Data points, for what they're worth:
- I use the same length for both SL and GS that I have used for years and years for ordinary skiing - which is consistent with the old rule of thumb (approximately). My theory (which I've actually tested, a little) is that I don't train enough to develop all new habits.
- Some people suggest longer for GS, with some logic. If you scarcely plant a pole in GS (some people don't, some do), the only time the length really matters is pushing out of the starting gate, when a longer pole gives you a little more lift and leverage.
- Some people suggest shorter for SL, though now I can't exactly remember why. I actually tried running slalom gates several times using a pair of poles 2" shorter than usual. Didn't like the result -- it felt a little funny, and made me drop my hands and get the inside one back a bit.