What shutter speeds are you getting in similar focal lengths? If 1/EFL or faster, go with the 2.8. If 16/EFL to 1/EFL, go with the 4IS. If slower than 16/EFL, get a tripod.
I/we own a 2.8 IS (v1, not vII) and a 4IS. My wife usually has the 4IS as she prefers the lighter lens. If she isn't shooting and I think I'll have enough light, I grab the 4IS. I'm keeping an eye out for a used 4 non-IS to have at my disposal.