When I first heard about the different factions in Destiny I asked my fire team which one sounded best to each of them. The more we discussed it the more frustrated we got because we couldn't decide on just one for the whole team to play for. Then recently I heard a quote by Bungie saying something along the lines of not wanting to limit friends playing on the same side. After hearing that I pieced some things together and came up with a theory. Guardians seem to be more like bounty hunters then soldiers. Employed by factions for certain jobs, but not pledged to any one faction. The Guardian's allegiance is to the human race as a whole (exo and awoken included of course). So maybe it isn't a matter of who to side with over all, as much as it is a matter of who to side with for now...