PhotobyMayo, Welcome to the forum.
I have never used the 50D but once had an XTi. I now have both the 5DmkI and 1DmkIIn. Both will give better IQ than the XTi or 50D. May not have as many pix but the overall IQ will be better. I have put nearly identical images side-by-side from my 1DmkIIn and a 7D and while at 100% crop the 7D was a bit sharper due to higher pixel count the overall IQ of the 1D won out IMHO.
I don't know why you had focus issues with your 1D before. Mine has a better focus hit rate than my XTi or 5D. I also used the XTi for studio work back when I had it. I once had one of my shots from the XTi up on a billboard around town. It looked great. If upgrading I would however recommend something newer like the 6D or 5DmkII or III. Not only will you get better IQ than any of the other bodies mentioned but also AFMA(Auto Focus Micro Adjustments) plus other enhancements. Shooting your XTi or 50D in the studio at f13 should not be a problem. Any diffraction will not be noticeable if your focus is good except when pixel-peeping.