they both have their advantages and dis-advantages... but unless you get some uber-huge image, on some crazy assed bumpmapped model with 40 bazillion faces, they both, are about the same...
Since spring uses relatively low texture memory other than the actual game map surface itself, the difference in what imagetype gets better performance is about +/- 5% depending on the machine, how its used, and videocard.
honestly, I'd just use whatever format you feel more comfortable with and you like better.
as far as what to do, there is no "rules"...
if you want to make a map, and have some naked girl on it, go for it, its been done though..
if you wanna make units, go to town, there are quite a few modmakers making entire armies of units right now...
If you are a texturer, and you honestly are, I'd have more than a million models for you to skin in a few hours

... naw, but seriously, there are lots of us content creators that are better at modeling than skinning, and frankly, we can use all the help we can get.
If there are downloadable links to any models I've made, and I hope I speak for others as well, and their models, go ahead and skin them and post them on the forum. Just always give the creator some credit and try not to say your work is "better"

I got cranky one day for that and threw a bit of a fit, not one of my best days...
If you wanna make features, go for it, I can use all the help I can get with making content for maps.
maps themselves, sure, just avoid anything "speed*.*" or anything thats a "take the middle" map... and if you DO make maps, put frakkin features on them
and if you are a coder, you are a saint sent from the gods! help them fix the damn de-synch issues, they popped up around spring version .64 if its any help, and honestly I think they happen from pathfinding and mass command issuing, I.E. when a player tells 400 someodd units to do the same task, or series of tasks.. (too many commands at once and basically floods the input/output udp ports.)
If you'd like to just re-vemp the wiki and make an upload section or someting, hell a real directory or contents page or something... thats cool too...
there is just a ton of things to do, jump in, have fun, and if you get tired of doing one thing, do the next, it never hurts to bounce around...
myself, I bounce from texture/model/map/features/suggestions/ to mentoring the new guys... I dunno.. sometimes I'm an A-hole too... but hehe we all have our moments..
have fun!