IF I had another 5 bills I would get the IS. However, I'm gonna go with the non IS L f2.8. The IS is a dream lens but just can't swing it. I'm stretching a bit as is. Not real concerned though, canon lenses hold a great resale value and when I'm able to jump up, it will be at a minimal loss on resale.
Most show photography I do, the lighting is really good. If not, I can always jump to my 1.8 50mm. or bump up my iso. At worst I will pick up a monopod which should be plenty of compensation (and cheaper than the jump to the 2.8L IS).
But again, most of my shooting is outdoors, of people and landscapes. The 2.8 is needed for the sake of people and the closeups of plant life, so as to really blur out the background.