I don't know that I'd bother with a Scrapper, especially if you're new to PvP. There's definitely some good ones out there, but it would be one of the more demanding PvP classes in terms of finesse. Even still, scrapper scoundrels/concealment ops are sub-par until 36, and that's a whole lot of time to be lousy at PvP (if you plan to do it while leveling).
Then again, the commando isn't a much better choice. If you're picking between commando or scrapper... good luck. I would suggest going Sniper if you want ranged, or Mara if you want melee (from the choices you had in your original post). Maras/sentinels can do fairly good damage early on, get their defensive CDs spread out among the levels (so you feel like you're constantly improving with your skill choice), and the same for Snipers/Gunslingers.
If you're new to PvP, I'd definitely suggest one of those or a PT (but you already have one). PTs are extremely simple but effective (but I see you already have one, so you might want something new).