IMO, the difference between standard developers, while generally visible in some way, is effectively insignificant for "general-purpose" use, and totally overblown by most photographers (in general). If you can't get consistently good results in general-purpose situations with any standard developer, you aren't trying hard enough; aren't putting in the time and energy to really get the best out of yourself and your materials, so the choice of developer doesn't matter anyhow, and you might as well go to a lab.
AS for D-76 vs. X-Tol, they're both good, standard, off-any-shelf developers. I also don't have much use for either of them in what I do. For general purposes, I use HC-110. Its qualities generally suit my subjective aesthetic desires for my pictures, and it is as consistent, versatile, easy to handle, and long lasting a developer as I have ever used. For special purposes, I use D-19, and as of late, Rodinal and D-23 (both usually quite diluted).
I used to use D-76 for everything. It's good. I like its character. I don't like mixing it or storing it, or being unsure of how long to keep it. I think it is best made up batch at a time, and I find this annoying for a general-purpose developer. (If I am going to mix up my own, I make D-23, which is a bit more simple.) You can get D-76 in one-liter packages, but it kills a lot of its excellent economy (which is greater than HC-110, despite commonly-held beliefs) when you do this. I think that D-76 is best if you consistently process a very large volume of film. I never got the consistency of results that I get with HC-110, and to me, nothing is more important in a general-purpose developer than consistency. D-76 and HC-110 are the two developers I recommend to all beginners. They will always get the job done, and are affordable and versatile.
As for X-Tol, it is very easy to mix, which is nice. It works very well. I simply don't find its aesthetic qualities pleasing for most of what I shoot, and for my subjective taste of what my pictures should look like. On paper, if you want to choose a readily-available off-any-shelf developer based solely on technical specifications, this is the one. However, I find that it is so general purpose and so technically sound that it lacks the character I like and am used to from more "flawed" developers. I also find HC-110 more consistent, again. I have used X-Tol as a substitute for T-Max developer in certain cases, and it gave me great results, but for me this was a special-purpose situation which was well-suited to such a "technically good" developer (flat light, T-Max film, overdevelopment, wanted razor-sharp, low-grain results and lots of support in the low tones), and don't see myself doing much else with it other than this in the future. My overall impression of it is that it is kind of bland for me, and also that I like its results best with T-Max, Delta, and Acros, not any of the random-grained films like FP4, HP5, Tri-X, etc.
So, while any of the three developers above can certainly be used as "general-purpose" developers, I only view X-Tol as a "special-purpose" developer for my own purposes. No reason, other than a subjective opinion of what I think my pictures should look like (in general).
So, the choice between X-Tol and D-76 comes down to what you want in your pictures.....and is subtle, though noticeable. I suggest trying some of each in a controlled comparison test, and seeing what happens. As I said, for "general purposes", I doubt you will see any practical difference.