Sparky,
Take my word for it, once you try Rogers technique, you'll never go back to Harries. That means that I'll recommend against the 6P and G2 lights that have been recommended so much here. Just having Harries as your only option blows, and there's no reason to limit yourself to that if you're looking at the Surefire line. The Z2, M2, and (I think) C2 all provide for Rogers technique, as do their 3-series siblings (Z3, M3, C3). You can use Harries on these lights if you decide you favor that, but you also can check out Rogers technique, which you'll almost certainly favor!
[edit]
I just read Dano's excellent advice that you should pick a light that provides for a hold that the instructor will talk about. However, the M2, etc., *do* provide for the standard Harries technique that everyone with a 6P, Asp, Scorpion, etc. will use. In other words, M2 and the others can do Harries, but only M2/Z2/C2 can also do Rogers.
When I went to LFI, Mas never actually got to any flashlight shooting. However, Mas doesn't appear to be a big fan of Harries, either, and it's almost inconceiveable to me that he isn't aware of Rogers, given how many Surefires are out there.
Bottom line: I'll repeat my advice that you absolutely should get a SureFire light that gives a Rogers option. Top choice (and most expensive) is the M2, then C2, then Z2.
Joe