Well, have not had any of these frames or even any of the brands, so will just give you my take on them.
Surly is a brand owned and distributed by QBP, just a basic steel imported frame from asia at a good price point. I have a few friends that have built up Surlys and like them quite a bit. They seem to have quite a following these days.
Sette is Pricepoint's in house brand doing much the same thing but with an aluminum frame (and I believe now offer some carbon frames). I've read some decent things about them, they have their fans.
Gary Fisher is the Trek division pushing 29ers more than the others, and GF's take on geometry (Genesis/G2 or whatever they call it these days). X-Caliber is mid lineup I believe, but just another basic improted aluminum frame really, nothing all that special. Generally the most expensive of the three new. Huge fan base, have a few friends who ride some, they seem to like them fine.
I'd take the Surly because of the material alone, don't have any desire for an aluminum ht frame and wouldn't mind the weight penalty. The most interesting thing about any of these three would be in how you build the bike out...whatcha going to put on one of these frames?