Well personally overall lore wise I find chaos more interesting. Plus slaanesh being the god(dess) of doing what ever you enjoy however you want and as long as you want. If I was an imperial servant (human or space marine) I would fall for him/her in seconds. Considring life in IOM is pretty much a life of denial and all abandonment of personal freedom. You are simply a cog in a machine thrown away and discarded when you die and no one would care or offer you anything in return for your work. Chaos is the opposite. With chaos when you die you don't get torn to shreds/tortured/violated in the warp like everyone else. You become one with your patron and if you are a champion of note you are used in the great game(according to liber chaotica) or if you are lucky immortality to serve the god(dess) you love most.
