Play each one to around 20 and see what fits you better. no rush in this game imo. For the record i got a 50 merc with dual champ pistols and let me tell you, if you slip under the radar and are allowed to dps unmolested, you can seriously tear stuff up fast. Heatseeker+tracer+unload+explosive dart and most guys are close to 50% if not below. Now if people start figuring out that you are a threat in pvp, well you're gonna get focused and then it kinda sucks
. Altho PT does seem kinda fun too. Been thinking of leveling up one of those too since my merc is pretty well full champ gear and pvp level 50 so really not much to look forward to atm other than level 65 for the speeder.
In the end it really just boils down to how you interpret what a BH or Trooper should seem like....as well as their roles. PT is a tank/melee dps, merc is ranged dps/healer.