It seems like scores on the WISC trend a bit higher than scores on the SB-5, at least a certain sort of kid, so waiting for the WISC might be better than either of the options you're considering. This was the case for us. (And this is purely anecdotal, so feel free to disagree. I'm just throwing it out there...)

But if you don't mind re-testing on the WISC in a year if the scores don't fit what you know, then I don't see a problem with testing at 5yo.

Kriston