I always kinda figured the difference would be a tactical thing. How long you wanted to be out of combat when reloading. Like the mag version taking 2 seconds to reload vs 4-5 seconds for the belt fed which leaves you open for long periods of time but gives u the option of continuous fire.

IMO, if a Heavy bolter dev' is on the field he would only need to reload when that huge back pack is out of ammo, and in such a case that HUGE back pack has to be taken off a new one to replace it should take a fair bit, when I was over seas I had a 400round 7.62X51 molle clipped pack on the back of my armor, it was 40,1lb(the weight reminded me how much that sucked) and I used if for dismounted everything, and when I ran out I was out I had my M9 and 3 clips but big deal, no more 7.62 means no more fire support. people may say oh well why didn't your AB or AG carry tmore ammo, to which I point out that there were SO many times that my AB or AG was some were else and after the first few months I lost my AG he became a driver and my AB as also a Designated marksman so he had his M4 mws and a M14 so all that ammo he already had tied into different jobs, I had no MGteam support but that is life as a infantryman you adapt.

In the grim dark that is 40k the nice thing about space marines and there bolter rounds is there all the same from the pistol to the Heavy bolter and the Boxmag from even pistols to a storm bolter can feed a Bolter or Heavy bolter all the same, now I know that they are different sizes on the model but I have read a lot of stuff supporting this, and that there is a different profile for the HB but I can attribute that to longer barrel and higher rate of fire, if you ever look on Marneus Calgar his gauntlets of Ultramar have arm mounted Bolt pistols that are belt feed, and as much as I want that level of customization in EC I will bet $10,000 it not going to be, hell they probably will not even allow for arm mounted weapons at all even though its cannon, fluff and Rules supported.

Another reason I like and dislike space marines they have SOOO much going on for them its nearly insane at times, from all there Gene seed enhancements that make them great to then say that woman cant use them?, What cause they got a Rib and different pluming BS, is a perfect example.

IMO
The weapons should come with Box magazines as a standard, the Belt feed should be a upgrade.
When Box loaded you should carry a couple off magazines with you and maybe have the option to carry extra magazines instead of other equipment. The Belt feed should have no option to reload because all the ammunition comes from the same box on your back, you don't carry several of these backpacks with you. both Box and Belt should be able to reload from resupply boxes. i also think that the heating problem should apply to both version.