Lets not start the 8 vs 16 man difficulty argument again.
It kinda depends on the guild. In WoW i loved 25m for the challenge, but found 10man to be a more relaxing and more social (talking with everyone a bit more). Especially as a tank who often had to lead stuff, 10m was more relaxing.
In SWTOR, I think 16m is probably more fun, there's usually more joking around etc. But that may be down to being in a very fun social guild
The fps drop is not so great though.