If you can afford it, it's always better to have nicer glass. The 50D will give you better burst speeds and faster AF, I believe, which would be good for shooting track and field. I have a 40D and it's awesome for sports when I can just shoot off a quick burst to catch the action. AF speed and accuracy are also pretty good. I can only imagine that the 50D would be even better in those regards.
Then with the extra cash you can pick up a better lens. If you heavily crop a shot from a 70-200 f/4L, it's still going to look a heck of a lot better than an uncropped shot from your 70-300 IS. Focusing will also be much faster and more accurate.
With the money you save from getting the 50D over the 5D and selling the 70-300 IS, you can also pick up an ultra wide angle lens for landscapes in addition to something like the 70-200 f/4L.