Well your opinion is wrong. 9400m IS supported by openCL, 2600 IS NOT.
Anyway, The newer machine has better, faster processor, ram and video card. Since it is more "future proof" it will last you longer than the other one. Also, the resale value will be more. Basically theres no reason to not buy the best machine you can afford. The older machine might run fine now, but once Snow Leopard comes out, the newer machine will run circles around the older one.
My post was posted before we knew what GPUs are supported. But anyway, we do not know how much OpenCL can really boost your system. I think it won't very much. Only Apple's applications will support it when it comes, so...