We went from a 2004 MPV to a 2006 5. My wife jumped at the chance once she found out the 5 was available. She tolerated the MPV but never really liked driving that thing.
The MPV is significantly bigger. The seats are bigger to the point of stretch-out room and the cargo room is much more immense. Certainly if you value extra room, stay with the MPV.
On the other hand, if the MPV is more room than you really need, particularly on a day-to-day basis, the 5 wins hands down. Significantly easier and much more fun to drive, and much more efficient. I think the 5 is a perfect size for two children, and there is plenty of cargo room except for serious cargo-van hauling. Don't forget the third row folds down split, unlike the MPV. This enables us to carry long objects by folding down one side and putting one child in each of the second and third rows; we've carried everything from bookcases to a file cabinet to Christmas trees that way. However, if we had three kids I think we'd move up to Sienna/Odyssey et al.
Regarding skis, with second row in use and third row folded down, they can fit extended between the second row seats. We don't have a ski rack for our 5.
I know what you mean: the 5 does seem small compared to MPV at first glance. But we don't miss the MPV's extra room at all.
