I'd upgrade
Mainly because I can't call myself a geek if every computer I use doesn't have the maximum RAM fitted.
But I also have this feeling that newer versions of operating systems use more RAM and RAM for older machines will become harder to find and get more expensive as demand drops. So you may find over the next couple of OSX releases RAM usage goes up and you may regret not doing it now.
My 15" Penryn MBP is about to go to 4GB with the expectation that, like you, I will still to be using it in three to five years time.