I haven't met a real person with the Dillan spelling.
Dylan is, I suppose, the "right" spelling.
This name is not on my faves list, so I am undecided about how I would spell it, as my subject line demonstrates. Given the somewhat dismaying popularity of
Dylan, the fact that I'm a dyed-in-the-wool Texan, and that I lean toward all things cowboy, I'd probably go with
Dillon if I had to.