I would get the Q9550, if I was forced to accept your supposition.
However, the suppostion itself makes little sense.
Why would you not upgrade for 4-5 Years?
Buy for today, not 4-5 years.
If you by something such as the Q6600 instead of the Q955, you will save about $300 which will give you more than enough money to upgrade again in about 2 years.
Anything you buy today will be seriously outdated in about 1-2 years.