well im not 55 on merc yet, lvling a 2nd sorc , sorc and merc are both 45 now, im finding my sorc more enjoyable, more burst, better aoe, (lightning spec is ridiculously fun because of instant chain lightning procs) in the end its the play style you choose, sorcs are squishier, but have a decent amount of escape tools and such (aoe kb that roots, stun bubbles, force speed, force barrier) i just decided last night to stop lvling the merc and focus on the sorc, i enjoy sorc healing too from time to time, not so much merc healin, i would suggest to level both eventually, and see how they pan out end game, im pretty sure they parse close to the same sustained damage end game
their cc is basically the same, also personal choice is i like sabers over guns, but umm yeah, you pretty much can never run out of resource dpsing with sorc too, thats a plus