I don't know it's hard to say... since there's many factors involved here
It might really be a toss up:
2. You're now at f/4 which disable the 5D high-precision AF sensor so might not be as accurate as an Xs on f/2.8
the sensor is only part of it. I realize you don't use focus tracking but most sports shooters do. It appears Canon went with the same focus system as the xti - which is much worse than the 5d. So, even without high precision the focus tracking of the 50d will likely outperform the focus tracking of the XS. For example my mkIII will track a whole lot better with a 5.6 lens than my 20d did with a 2.8. In my experience the high precision comes more into play at low light levels which shouldn't be the case for the sport under discussion.
Still, as I said neither is a very good solution. The real solution is to buy the right lens for the job. Either solution under discussion is not great.