I had been looking at the 503 and the 501 too. The 503 was a little out of my range, but the 501 didn't seem to have all the features I wanted. Then I saw the 501HD. I played with it and was blown away. The price seemed fair, and it felt like a huge step ahead of the 501. I bought it shortly after and have been very happy with it since.
If it's a question of price, I'd suggest the 501HD. If that's not a a huge concern, then it would probably make more sense to go with the 503.
503 is the minimum you want in the long term. If you simply cant afford it check out other options before the 501. I think a libec ls22 is a better unit than the 501. Neither can do much more than the camera alone.
ncje are you talking about the regular 501 or the 501hd? And from what I hear, it sounds like the 501hd is better than the 501? So the official ascending order is: 501, 501hd, 503?