Well, it's kind of hard to tell if the results are biased. If Underworld wins, it might be because there are so many Sahara types on the board and they don't like Tool. They're both old bands and I have a feeling Tool will be around longer so you'll have more chances to see them, because I read somewhere that Underworld wants to pass on the name when they retire...Don't know what that will mean in the near future.
But if you truly will NEVER see either of them again, I've seen both and was blown away by both. Underworld was a show I went to by myself and didn't dance but loved it, so the instrumentation isn't much of a factor. But then I was blown away a lot more by Tool.
Lollapalooza also has a fucking ridiculous lineup so that should make the decision finalized - go to Lollapalooza.