Congrats!
I have the 18, the 36, and the grandmeister (all heads). I think they sound great. I prefer the 36 for the extra channel controls and midi, but the 18 still rocks. You shouldn't need more than 18 watts. I play large clubs, and have done unmiced indoor and outdoor gigs. I still have never used the 36 watt modes at a gig.
Enjoy!