I think engi or medic is a really good starting class. You can get a feel for the game and earn some certs by repairing / healing, so you don't need to actually be able to get kills to be helpful and advance. Also, everyone in the game is very fragile, and it's best to approach things that way whether you're Heavy or not. MAXes, for example, need constant support from Engis repairing them - without that support the MAX can't remain in a fight for very long. But with it, he can survive indefinitely.
Any class can win a duel but a Heavy with his special shield up might be able to kill his target without taking any health damage. When his personal shields recharge he's ready for another duel. Non-heavies will probably take all their personal shield damage and then some health damage, and then their personal shields recharge. That means for the next fight they're injured and may die when they would otherwise survive with just a little health left.
Then again a Medic can use a healing aura thing to recover, and an infiltrator with a cloak might be able to down his enemy without being attacked in return. So each of the classes has benefits in a firefight. You could look at it that an Engi, because he was able to repair for his teammates, was able to keep them up longer, which meant they killed more and absorbed more hits that would otherwise be focused on him. The end result is fights won and territory claimed.
I've never played Light Assault so I'm not exactly sold on the jetpack. I see a lot of LAs trying to jetpack into places and getting shot down out of the sky, or otherwise just noticed because they're doing something weird and blasted soon after landing. The amorphous blob of enemy troops tends to react to notifications in the rear of their base, such as an LA capping a point. You do see some LAs floating over to drop bombs on enemy sundies. Not sure whether it's worth being an LA instead of, say, Engi or Medic. It's probably highly situational like any of the other classes.