they're mostly the same, but the exterior is different. A07XL is much nicer like aviationwiz said.
also the exterior case of the A07XL is designed differently, is supposedly made in such a way as to reduce sound.
The DVR-A07XL also incorporates Pioneer's new "Quiet Drive" technology. Part of this new technology is the honeycomb design you see in the picture above. According to Pioneer, this design helps reduce vibration caused by the drive mechanism. When combined with the DVR-A07XL's performance adjusting firmware and sound proof pads, this honeycomb design helps reduce noise by up to 75%.
(Ian's review http://www.cdrlabs.com/reviews/index.php?reviewid=225
if you paid for the XL model, you should definitely demand one.
if you paid less for the OEM 107D, it's up to you if you want to spend the extra money and get the extra features. (and the extra hassle of exchange)
as far as functioning as a DVD writer, they're the same.
One Ring to rule them all, One Ring to find them,
One Ring to bring them all and in the darkness bind them
In the land of Mordor, where the Shadows lie
M.C. Escher - Reptilien