50w heads could be used in ANY gig situation. 100w is only a third (1/3) louder so it may be psycological more than anything.
Personnaly I'll go for a Engl Screamer head, very versatil and powerfull (on a good cab, vintage 30 got 100dB sensitivity, enough to boost up the 50w :) )
Sure you wont have the "I have big balls and you havn't!" factor :lol:
JCM900 is just plain cr@p IMHO, as most of the newer Marshall...
If you can find a modded JCM800, just go for it!!!