I had to make the same choice and decided to get the $299 version.
It's my guess that most of the $299 versions have not been tested for overclocking ability, but there's a chance of getting one that failed at the overclocked speeds.
I figure the $319 versions have been tested, and probably failed the superclock speed.
The superclock version is out of my price range so I didn't even consider it really. And if I was going to pay that much I'd want 512mb of memory.
Another thing is, I doubt that Evga tests for overclocking at higher voltages, so that's another reason it's possible the $299 version could be better than expected at overclocking.
Plus if I cared a lot about overclocking I'd rather spend the $20 on an aftermarket hsf.