I haven't used a DVX100, but the most obvious difference between the two is the interchangeable lenses. If you have money to burn on new lenses and intend to use them, then the XL2 is really your only choice.
There's an interesting article here:
http://www.dvxuser.com/articles/shoot3/
which has a lot of comparison shots between the cameras and the Sony FX1, though obviously it's rather biased towards the DVX100
. That should be a good starting point for comparing the two.
All that said, you should seriously think about HDV as well: after shooting it for a while, I can't imagine going back to shooting on DV now. It's also worth mentioning that while the FX1 has worse performance in low light with 0db gain, as demonstrated in that article, in my experience the XL cameras look worse with 6db gain than with 18db gain on the FX1/Z1: don't know how the DVX compares.
Also, for all the talk in that article about how the DVX100 looks like film and the FX1 looks like video, I honestly can't see much difference between the side-by-side comparison shots except in low light. The DVX and FX1 look very similar in color response, while the XL2 has a different color cast (which, I must admit, I actually prefer).