If you plan on or intend on doing any tanking then PT is the way to go. They still have good DPS as well. If you want straight damage or don't mind tossing the occasional heal the Marc is the way to go. I have one of each and like em both.
Good part of each is that if you get tired of 2 hour FP queues, you can always respec to heals on Merc and tnk on PT.
Helps to have a spare set of gear around