Mark,
This has been discussed several times on here...usually with pRiCe CuBe providing the 'voice of authority" (mainly becaus he IS the man...listen to him).
Both are nice. I prefer Canon myself for all the reasons stated above. I stongly agree about future lens purchase driving the decision. I have a 5D and an XTi. I also second the 24-105 IS "L" lens by Canon...on of the best all-around lenses around $1000 out there. The Nikon has the new (July 09') AF-S DX NIKKOR 18-200mm f/3.5-5.6G ED VR II that's a little less...but I have never used this lens. Since both of these are crop sensors you would get a 28.8 - 320 on the Nikkor and 38.4 - 168 on the canon...not exactly an ultrawide lens. I have used the Canon 10-22 on the XTi and liked it but it is expensive. I found a Tamron 19-35 for my 5D that is pretty good (and I got it for $85 used..even better). Again, Nikon has a VR version (17-35) but it's in the $700 range.
You can also find the manual focus 8mm "Preleng" and "Bower" (?) russian lenses for both Canon and Nikon that will not break the bank. Stick them on even your 'crop' cameras and you have a decent 16mm wide angle (albiet a little 'old school' as far as feature sets go).
Go to the local Best-Idiot(buy) and play with them (or equal models). They have VERY different UI's. That may be the deciding factor.
If the camera was just for you and you were serious about the ultra wide end I would reccomend ONLY looking at Full Frame versions from Canon and Nikon. Costs more up front but you get a LOT of pluses for going that direction. The main one is NO CROP. Which means you don't have to over-invest in the silly-wide lenses just to get decent-wide pictures.
If you have no lenses, no fanboy tendancies, and no immediate lens upgrade needs...pick either one. I think you will be happy with either one.