Seeing as the DW series is WAY more popular in Japan than anywhere else, and the 360 is dead in Japan, I'm not surprised that XL is PS3 only. Heck, I'm more surprised that DW7 came out on the 360 honestly.
To the casual observer I'm sure there are no changes from game to game in the series, just as I would say there's nothing changed in Madden each year since I care jack squat about football. But they definitely do change things up gameplay-wise in DW - sure, you're always slashing hundreds of people, but the systems at work around that change up from game to game. At any rate, DW7 is the best one so far imo (haven't played the XL add-on yet).