So I started playing in a band with 100% fully structured songs on bass the end of last year (
www.myspace.com/circlepit) after having only played in noise/drone/noise rock bands before hand. I can honestly say it's much less challenging (at least in the circumstances I've been in) playing in a rock band than playing improvised music.
I've switched over to synth (Realistic MG1, Moog made for radioshack) and some other feedback fx in my "rock" band, which is a pretty similar set-up to when I play "noise" (add some cassette loops) and I'd say its still more exhausting to play a noise set. I'm not talking pure-energy Masonna style stuff either, more mid to slow pace, building and changing slowly.