Actually Ikelite housing for 20D and Rebel XT cost exactly the same. So if you go Ikelite route, there won't be any difference beside the cost of the camera. However with 20D, there are a lot more choices, granted generally more expensive choices, such as Subal which offer aluminum housing with an option for a large magnifying viewfinder etc. Rebel XT viewfinder is the same size as Nikon D70 which is quite small and sometimes a pain to use underwater especially with your mask etc in the way. One dial each for shutter and aparture control rather than one dial for both is also very handy for quick adjustment underwater.
I personally happen to think that the extra money is worth some of the extra features in the long run.
I personally happen to think that the extra money is worth some of the extra features in the long run.