I only planned to get to lvl 3 on all the factions, what do you get for going higher?
Every rank gives you a legendary, either from the faction's pool or ascendant materials, I don't think you can get strange coins or motes of light though (at least not in place of a legendary piece of gear, I hope), you may have gotten one from the XP you get for opening the gift? Also the Crucible factions give you guns that you can't actually buy from them, I think it was
@Terronium-12 (?) who got a New Monarchy (?) hand cannon (?) and I got a FWC fusion rifle. I think that might mean that all five factions have a full set of 12 weapons, but only the Vanguard and Crucible quartermasters actually stock them all.
Like, from the Vanguard I've had a legendary scout rifle and machine gun as well as a cloak, from FWC I got the fusion rifle and from New Monarchy it was the Vanquisher VIII auto rifle.
In my opinion the best thing to do is rank all of the factions up to the third then focus on the ones with the small inventories since you can buy weapons from the other two. Actually, Vanguard is the worst one to work on because of the fact that you can only have 200 Vanguard marks in your inventory at a time, so you'll end up
having to buy things to spend the marks since you can't spend them elsewhere! Actually, the whole thing with marks - as it stands, this may change - seems really shortsighted anyway. Once you get to a point where you have a full set of legendary armour and enough legendary weapons you don't actually need to buy anything with marks any more, but then they do become a source of guaranteed ascendant materials so I guess that's good.