The HD-A2 is adequate to my needs, but doesn't provide as much capability for the future. The XA2 is way WAY overkill for my current needs, but may be more in line with future needs.
I'm still hesitant to buy in before the format war has a winner, but I already own a set of HD-DVDs, and REALLY want it to win over Bluray.
addendum: I actually already /bought/ an A2, back when there was a good deal going on. And I sent it back because they screwed up the order and I didn't get the deal. It's not longer available from Amazon.
edit: LG has a dual-format player (hd-dvd and bluray), but it's $700ish.