WISC is definitely your test, no doubt about it. I'd be very surprised if anyone recommends anything else. On the bright side, this is also the cheaper of the two main tests, since it requires a lot shorter time to administer.
My one worry with the cheapie student testing is that they might be looking for LDs, not for GT. You want to be sure they don't start the test too low or stop too early, that the tester is aware of GT behavior and knows how to get the best, most complete performance from the child.
I think if you can get this from a student tester, then more power to you, but I also think it's harder to get these things from someone not experienced at GT testing. It's a trade-off.
Worst case scenario, you try the student testing, and if you don't think the results are accurate, you test again with the psychologist. <shrug> It's a minor gamble, but if the first test gives you what you need, you save yourself the trip and the cash!
