Both of them hold their value quite well, as both of them are in Apple's premium lines. I sold my 2.5 year old 08 MBP for about 50% of the cost a couple weeks ago, and I constantly see 1st-2nd gen MBAs on craigslist for $800, which seems pretty good.
Or you can think of it this way if you're thinking of selling in a year ... the MBA just went through its big redesign (and bonus price drop) while the MBP did not. Given the spec that the MBP is due for its redesign next, I'd step into a MBA to avoid the risk of having an old design MBP in a year.
Thats kind of hard to answer because it depends on model, how old it is, is there a refresh coming soon, etc.
Even though I would like to think its the Air, its probably a MBP which would appeal to a wider range of users and therefore more potential buyers.
I was thinking the MBP because it's user upgradeable, but I would LOVE for it to be the Air. I have a 2011, yes 2011 MBP and I think I'm returning it for the Air. I just think that choosing between the 2 at the same screen size, the Air appeals to me more because it is lighter (much lighter), faster because of SSD, and has a matte screen with higher resolution. The Pro benefits from the Core i5 processor and the backlit keyboard, but I don't do anything that is processor heavy, at all. I do light things like web browsing, Microsoft Word/Excel/Powerpoint, use iPhoto, iTunes and I use Parallels for school but not too often. All in all I'm going to benefit more from the Air, but I'm hoping I can still sell it for around $900-1000 when there is a refresh because I do like having the latest and greatest.