I don't have either DRG or MNK at 50, but the core group that I play with runs both in 8 mans.

MNK has higher personal DPS, but requires more time to build up and loses a lot of DPS anytime he is forced to lose his stacks of Grease Lightning. You probably played enough PUG to know that the MNK is all about rotating their stance skills to eventually build up enough Grease Lightning, and then maintaining it while keep stance changing.

On the other hand, DRG has a bit better burst and less start time for his DPS (or at least it feels that way to me). The biggest plus for a DRG is his ability to increase piercing damage done to a target. This means if your group likes to have more than one BRD, then the DRG would be significantly increasing the damage of the group. Since you have a level 34 DRG, I don't need to tell you about the playstyle.

Overall, play what you want. Both are good in 8 mans.