I've not worked much with the 500's, but the only issue is that the reflex mirror doesn't show full frame on some lenses due to size/vignetting issues. I have the 501CM and it's the machine (together with an SWC) I intend on keeping for a long long time.

As for the 503, I wouldn't bother putting in extra money in for it, if budget is tight. I do studio with the 501 just fine. Lenses, on the other hand... I would spend a little more in order to get into the CF range.