The Unibody, new trackpad, new keyboard is reason enough to upgrade.
What you are buying on seems to be cost, you have a ceiling of say 13 or 1600. The biggest difference between the new MB and MBP's is screen size. I think the new ones are the only way to go, even over the early 2008 MBP's.
Personally I would decide if you wanted a 13 inch, or a 15, depending on that I would go new. If you like 13 then it's easy since you are already considering it. If you like the 15 then I think your decision is a bit harder, and personally, though you can get an old gen for a great price, the new ones are worth the money (magnetic latch also!). I would buck up and spend the 2k on a pro, or wait for black friday and spend 18 or 1900 on it.
Good luck with your decision.