hello Transmogrifier, I see you are still having second thoughts about this decision.
Well, first of all, at least you can consider yourself very lucky in that with Clinton Portis, you satisfy both needs (long term and this year). Some people won't even have that.
Well, in your case, where you guys keep two players, I would keep the guys that give me the better chance to win THIS year.
Why is that? Well, every year in football guys emerge from mediocrity or even from nowhere to become stars. The big names in the game are always changing. I don't know how long your keeper league has been going on, but think about it this way. With only two keepers being kept from year to year, I predict a fairly high change in keepers from year to year.
For instance, last year I doubt Deuce McAllister, Michael Vick, and Clinton Portis were keepers, and yet they have become great keeper options for this year. My point is that more guys will emerge this year that will be considered keepers for next season, and they are still available in your draft. Usually it is the younger guys with upside, like Willie Green, TJ Duckett, or Trung Candidate.
So no matter who you pick as your keeper for this season, you might even want that person on your team the next year, instead you might keep Portis and Duckett (or something like that).
In a league where lots of guys are being kept, I look to the future, but in leagues where few are kept, I am almost always looking at the right now aspect.
Even with all that being said, I know you have a tough decision to make, I wouldn't want to have to choose between Ahman Green and Donovan McNabb.
I would still take Green though, and go all out in your defense of the title.