I've been looking at canon 70-300mm f/4-5.6 IS USM and the canon 70-200mm f/4L USM there is only a $50 difference.
I tossed up that decision for ages, ended up with the 70-300nonL, used it for a few months and came to the following conclusion:
on the 70-300, the 70-200 range isn't that bad, only slightly behind (sometimes just in front) of the 70-200 f4L nonIS in resolution. Plus you get IS, and it's a good implementation. And neither are weather sealed. the 70-300nonL is lighter and packs up smaller, using a polarising filter takes getting used to (front rotates when focussing) but isn't impossible.
It's really let down in the 200-300 range though, fuzzy at 300mm, which is where i really wanted extra reach, so i got the 70-300L when it came out.
For outdoors, and for horses (which are huge compared to the birds i go for), you probably won't need the extra 200-300 reach, and if it's well lit and sunny, staying at ISO 200 or so, you can easily get good handholdable shots at f4 with a fast enough shutter speed, you won't need the IS (although it is handy for framing). f4L nonIS also focusses faster than the 70-300nonL (USM vs micromotor). So i'd recommend the 70-200 f4L nonIS.
if you want to spring for the extra cash, the 70-200 f4L IS and the 70-200 f2.8L nonIS are both about the same price as each other, neither will disappoint you (read the reviews of the f4IS, it's one of the sharpest lenses you can get for the price, and weathersealed).
One thing to watch out for, make sure you constantly adjust the EV when shooting, if it's a dark or light coloured horse, the camera will try make the dark horse brighter and you could blow out your background (or just meter manually).