If you actually read up in the Grimoire on the factions: New Monarchy want to defend the tower, fair enough, but they want to do this by installing an actual king. What a bunch of retrograde, forelock-tugging, archaic fawning morons. A king? Really? Will his son rule when he dies? Who will his son marry to make the next king? (One of his sisters/cousins is traditional.) Stupid. Dead Orbit think we're going to lose, so plan to run away. Fatalistic emo cowards! Just bugger off to the reef then. Future War Cult think war's inevitable and want the best possible gear to fight it with. Finally a world view I can get behind! Get me that gear! Vanguard/Crucible gear is like the baseline that everybody gets, but on the whole it's better than the equivalent faction gear (with a very few exceptions). It's a missed opportunity that the factions' gear doesn't represent their world-view better. New Monarchy should sell good armour but rubbish guns. Dead Orbit should sell fast sparrows for running away on and FWC should sell awesome guns but little armour. So in the end I think people pick based on what colour shaders or cloak/mark/bond they want.