P35 boards overclock better than any SLI board...
but SLI has the future of expansion...
keep in mind though when you sell both cards its going to be hard when its outdated...
its really up to you... I recommend just getting the SLI board, with a 9600gt and eventually use the eVGA or BFG Step up program to get a GTX 260 or 280...
But honestly I hate SLI and prefer P35 boards