I'd consider smgs over assault rifles. There is no real incentive to pick bigger gun over smaller more comfortable one in airsoft, well except mebbe to hide bigger battery.
That depends on where and how you play, and the rules you play with.
If cramped speedball arenas or indoor ranges are all you can find, or want, an SMG is the obvious choice, it's also a good choice if local rules only permits M90 or M100 springs, since an SMG will do that well.
But if you play outdoor ranges, or even MilSim, the more accuracy and power you can get out of a Carbine or full leangth rifle is definately prefable, more so if you are allowed to use M120 or higher springs, where the longer barrels start to proove their worth.
I would recomend a Carbine to the newbie, if all you have is one AEG, a Carbine will let you play both in and outdoors, whereas an SMG would suck outdoors, or a longer rifle would suck indoors, so you get to try more with a gun at around a meters leangth (AK or M4 comes to mind, and indeed the G36C), folding stocks are also a good choice since it will allow you to "scale" more readilly.
Anyway, more on topic, i have lots of stuff, but since i am selling alot of it and getting new stuff in about 3 weeks time, i wont bother to mention it all, but when my new loadout is ready i'll gladly show of some pics.