Uh oh, I hope linkshell 'hang out spots' from the two servers don't intersect! That would be awkward. :P

To the OP: I think your 'best bet' is to really look at how you like each linkshell's attitude (like Catapult said), but also look at the time zone of the linkshell you'd like to join. There's nothing worse than not being able to play because you simply can't make events at the time everyone else is on.