I think Michael R 1974 said what pretty much what I think, but I'll reinforce that with my own experience.
When Phil Davis tested Xtol for Photo Techniques magazine he ran a D76 control with each film and the curves were very nearly identical (except speed point). I recently did a quick test with D76 that confirmed this to my satisfaction. I finished one roll of TX in a camera and started the next roll with the same shots. Ran the two rolls in parallel, one in Xtol 1:1 and one in D76 1:1 for the appropriate times. I got the gamma of the two rolls very, very close and the prints were nearly a dead on match at the same contrast. The only real difference I could detect was a slight increase in shadow detail with the Xtol (maybe 1/3 stop), and just perhaps a bit finer grain and sharpness with the Xtol.
I've used Xtol replenished for quite some time, but have recently gone to 1:3 one shot. This gives the same economy as replenishment, plus a slight boost in speed, sharpness, and grain. I plan to keep my replenished bottle going for now and will use it when I want the ultimate fine grain. The only disadvantage I can see to 1:3 is the longer time, but that is good in every way but convenience. The only other disadvantage I see to Xtol is the PIA of mixing the 5L size.
D76 is great, but Xtol is a bit better for my needs. Nothing at all wrong with using either. Maximum film speed comes with dilute Xtol, but either will push the higher tones well.