To trade, or not to trade....

I'M facing a dilemma at the moment.
As always happens this time of year, cash is in short supply, but as a keen gamer, I don't want to miss out on the latest big releases.
And with just Killzone 2 (a must-have) just weeks away, I'm pondering how I'm going to find the cash to buy it.
Now, my friend has made the bold decision to trade in his copy of Resistance 2 at Gamestation for a £20 credit note.
I have the same game, but am only a third of the way through the game and still enjoy the online aspect - but as has been pointed out to me, will I really play both games?
So, do I trade in a game I still enjoy for one I really want, or do I keep both, and try and find some other way of finding the cash? Ideally I would own both, but I don't think that's an option.
And if I do wait until near the Killzone 2 release, the trade-in value will go down - decisions, decisions.....