Bad Homburg has only 5/10 EUR and usually only one room running; imo don't bother going there during the week; games on weekends can be very good and often run two tables then.
Wiesbaden is for sure the nicer casino, has more tables (Only Monday 5/10+ thou; otherwise 2/4 only) and a much better "vibe"
Spielbank Mainz also has poker games, but havent been there yet; check
http://spielbank-mainz.de/ (but only in German as it seeems)