My first (and only) toon I got to 55 is my Commando, which is the Merc counterpart. The bottom line is that the Merc will use a lot more buttons to be as effective in PVP. While I do have a lot of tools I find my Operative to be a much better healer in PVE and PVP. If you are going for DPS PVE then Merc all the way. In PVP I just feel stronger on any other class I try. Granted none of them are at the level cap yet.
My PT is at 20 though and he will probably make it to 55 before my poor Force users. IMO the PT has the best animations in the game.
Oh, and I use Treek as heals on my Trooper for dailies. I can kill more things quicker and with more HP at the end than having Treek as a tank, and that includes Elite fights.
