Sorry it's my 1st post, so I hope that this doesn't flame anyone...
When I was deciding between the XPS2 and the 9880, I found the 9880 to be more futureproof with the 6xx-series processors (b/c of EM64T and HT). Correct me if I'm wrong, but I don't believe that the PM's in the XPS2 can be upgraded to 64-bit processors w/o a whole new motherboard (which I don't think Dell even does upgrades).
So in general, for the near-term the Dell is most likely the superior machine. I think Dell is also given a bad rap on their quality, as I had a Latitude before my 9880 that worked like a charm.
However, once Longhorn comes out, the 9880 can take over the XPS2 with its P4 6xx processors... So for me, the deciding factors between the XPS2 and the 9880 were EM64T, more RAM, dual optical drives, and SATA RAID.
Plus, it's not very often when I can game on the go with my laptop. If I would, it would either be on an airplane or on the Metro (which I doubt would be very smart

)
Just my 2 cents.