Ran: OP gears/char. more expenssive. 2nd most populated
Jin: OP gears/char. more expenssive. least populated
Isen: OP Temp. less expensive. most populated.

so if u r willing to spend lots of real money, go to Ran (since Ran is more populated)
but if ur cheap, go to isen and be a templar :v

*however things may change due to merge RanJin & upg.