had a Rebel T1i before this camera...I've taken some beautiful photos from this camera, but the shutter speed was annoying at times as well as other features I did not have. I take a lot of photos of babies and toddlers, and wanted a super fast shutter speed
Not sure I understand this. I have the T3i and up until last year I had the 300D. I have a 4 and 6 year old son and daughter and now a 1 1/2 year old super energetic dog. I have yet to find the shutter speed to be a limitation. Buffer depth in RAW, noise when I push the ISO and constantly fighting for more light (and pushing the ISO) yes, shutter speed, no. I have the opposite problem. It all depends on the type of photos your shooting, which brings me to my point.
Take a look at what you are shoot and what you want to be shooting. If you are routinely shooting at high shutter speeds, then the IS won't be of much benefit. You need to be 1 over the focal length on FF, so with the 70-200 F4 non IS, that would be 1/200 of a sec on the long end. If your needs will be at or above that, technically you should be fine without IS. The beauty of the 5DIII is that you can push the ISO higher than I can and get favorable results if you do find yourself in lower light.
No doubt the IS is better (the 70-200 f2.8 IS is even better, but it's $2200), it's what I want. But I have lower ISO capabilities until I upgrade (lenses first for me) and I am routinely in low light when I use longer glass. In your situation you may not need it. Besides, the $600 you'll save is a good chuck toward a 24-105 and if you do decide later to get the IS, the non IS will hold most of it's value. Just check used prices on them.
The other thought is that if you do decide on the 70-300L, the slower aperture won' t be an issue with the high ISO capability of the 5DIII and what sounds like good lighting that you are shooting in.
With such a snazzy camera, it would be almost criminal to saddle it with a 70-300 non L, but if your so inclined I have the highly coveted
75-300 IS that I would gladly trade for your 85 1.8.