So on a day to day basis, how much shit do the IG deal with and the SM deal with? Who's really keeping what little of the Imperium is left together? SM get tons of glory lore and big battles but is it really that important? How much does the IG deal with constantly before things require SM to get involved? And IG have had some monster battles of their own. WEIGH IN LORE GEEKS!
SM and IG have very, very different forms of heroism. The Imperial Guard have the heroism of common men and women standing up for themselves and, yes, doing most of the work of defending the Imperium. The Space Marines are fire from heaven, steel and doom to the enemies of Man, especially the enemies no one else can face (effectively). When the Adeptus Astartes make planetfall, as far as the common citizens are concerned the Emperor Himself might as well be defending them personally. Could the Guard handle everything themselves, given the appropriate resources? Possibly. But that's not the point. For a real world example, Lt. Col Hank Detering one said, "America doesn't need a Marine Corps, America wants one!" And this would be ever so much more the case if said Marines were gigantic supersoldiers with the best gear in the galaxy.
It's the argument of Big Boss and the U.S army. Or the entire allied war force in WW2 and a platoon of the best of the best. Or medieval knights and the forty men-at-arms they bring with them.
It's kind of a mute point. It's like asking which branch of a nations military is more important. The only difference is that in Warhammer 40k, the Space Marines would quickly be overwhelmed if the Imperial Guard didn't exist or were magically all wiped out at the same time leaving only the Astartes whereas if the opposite occurred, the Imperial Guard have the tactics, the manpower, and the leadership to hold the line. Remember this: The Imperium of Man is only HOLDING its own as of the 41st Millennia and while they do make gains, they are still unable to keep it or expand past their original Great Crusade boundaries. The Eye of Terror is a constant endless spam of enemy's which eats up the largest share of Imperial military might and the upcoming Tyranid invasion which from what has been established is merely a scouting force so when the true Tyranid host arrives, the Imperium of Man will probably last as long as the first encounter. For those that don't know, the first recorded encounter in fluff was a fortress world stationed with over 9 army groups of Imperial Guardsmen and heavily fortified. It lasted less than three hours, essentially that would repeat itself when the Tyranids show back up.
First encounters always go poorly. Its to show how super scary and powerful that new threat is. Since then however we have seen much weaker defensive forces beat back the bugs and even preform counter offenses. In the lore there is only one faction that could reasonably beat the imperium. And that is necrons. Hell they have an IPAD that they could just shut off and every faction but them goes poof.
Problem with that "theory" Tyranids are a wild card of unwritten lore. Not even GW has gone far enough to even mention what they have in their main fleets and almost all Imperial doctrine is scorched earth tactics with most revolving around Exterminatus which fails most of the time. If you look through the Imperial Navys records of combat actions, they barely have a 15% victory chance each time they engage and every time they take almost 100% casualties meaning that even though they stopped a mere splinter fleet which is the smallest of a Tyranid incursion, they aren't even holding the line, they're losing ground and tactical resources just to force them off until they come back. This goes against what the OP was talking about when it comes to Guard/Navy versus Astartes. Neither is capable of securing the territory, holding it, and pushing back to either keep what you had before or gain more so therefore neither one is capable of being a good counter force to the Tyranids. If it wasn't for free-lance writers and commissioned artists who undoubtedly show biased plot armour, the Tyranids would of long overrun the outer sections of the Imperium. Another factor is that every time a Tyranid splinter fleet has shown up, the Imperium has almost never been by itself. Normally there's another faction which ends up getting caught in it splitting the enemy host which again prevents either one from being an acknowledged force. Necrontyr are the same way. Their lore is so one-sided with a fantastic loop-hole of being asleep that if they showed up in force just like the Tyranids no one would be able to win. Warhammer 40k has too many problems with its own lore that contradicts itself to even be remotely balanced for a technically plausible case of who would win what unless you only locked it to the same faction against itself. It's been pointed out in technology, tactics, and even execution particularly when it comes to the Imperial Guard. For instance, take a look at most of the Leman Russ variants and then actually do the measurement. Not only is the tank too small to hold the ammunition it is claimed to hold, plus a maximum crew, the sponsoon mounts, and spare components most crew would carry, the tank is also insanely wrong when it comes to how the standard main gun works with traversing. They can only get away with it because it's science fiction but every person with a military background who has served in an armored regiment can easily see the flaws. It's one of the primary reasons I dislike most of the Imperial Guards ground vehicles because they're horribly inaccurate in design and scale.
An unpopular opinion that will surely get me a lot of flak, but: I think Space Marines are boring and one of the weaker elements of Imperial lore. The diverse tones and fluff of the various groups of Guardsmen makes them much more interesting than the typical overdramatic, pseudo-Roman characterization of the various Space Marine chapters in terms both literary and visual. Other than the Space Wolves and Black Templars (and a few others I'm sure I'm forgetting) most of the Space Marine chapters are palette swaps with minor fluff differences (how many of you actually remember that the Raven Guard and Iron Hands exist?) and I'm not even counting the various "who?" chapters who have absolutely no background other than being shown in a "painting Space Marines" side panel in White Dwarf or one of the older codexes. Meanwhile the various groups of Guardsmen have completely different cultures and approaches to warfare that are reflected in their equally diverse appearances. A story told from the perspective of a Cadian versus that of a Valhallan will have a drastically different tone than if one were to contrast the Ultramarines and Imperial Fists. And there's just something more human and romantic about tales of the common man giving his life in defense of his homeland as opposed to some godly super-soldier who overcomes all challenges with ease because he is written to be infallible. I think Eternal Crusade would be a much more interesting setting with more diverse gameplay if the Space Marines were swapped with the Imperial Guard. Guardsmen, Stormtroopers, Commissars, Ogryn, Ratlings, Priests, and Psykers clashing with Greenskins, Chaos Space Marines, and Eldar sounds way more interesting to me than having two playable factions of Space Marines.
Both are keeping what little of the Imperium is left together. As far as which are heroes are concerned, it would depend on what kind of heroism you're speaking of. Space Marines are heroes insofar as dragon slayers are heroes. They're the Hercules figure that enters the hydra's lair and kills it. The Imperial Guard are heroes insofar as a contemporary soldier is a hero. They're the heroes that sacrifice themselves to defend their families and peers from the big bad monster. If not, they're shot in the back as a coward. Heroes die by the millions. Demi-gods die by the hundreds. Life is cheap in the 41st Millenium.
