I think the popularity of the FWBs might be more down to supply than anything else though
WTSC's tried Steyr, Walther, FWB, Hammerli and my old Anschutz. The Steyr LG100s are grand little rifles, especially for smaller-framed people and the LG110 is the only rifle I've ever fired that has absolutely no perceptible recoil whatsoever; the Walthers are very good as well, though personally I don't like them (no idea why, they just never seemed to fit me right - but they fit Ray just fine, he's just set some new Irish records with one, albiet in an MEC Revolution stock); the FWB P70 and P700s are fantastic rifles and are still the main rifles on the international circuit (though that might be because they predate the other manufacturers getting decent rifles to the market); the Hammerli AR50 seems to have a good following in the US, but we had several AR20s and had to send them all back - wonderful stock design, nice balance, lots of promise to the thing, but the innards broke on all the rifles within a month of club use through busted seals or failed linkages or whatever. Can't recommend them at all at the moment, though I accept completely that that was some years ago and they may have resolved this since; and as to my Anschutz 2002CA, well, the platform's solid (Katerina Emmons won a gold in Beijing with one), even if mine is a Frankenrifle given how much of it I have replaced over the years. But ten years of use without servicing and it's still able to hold the ten ring with unbatched ammunition. That's gotta say something for it. They don't make the 2002 anymore, but their 800x and 900x series rifles have excellent reports from everywhere I've seen them used.
Basicly, with very few exceptions, almost any ISSF air rifle is going to be more than good enough; if you can, try holding as many as you can to see which fits you best (and I don't mean physically, because all can be adjusted to fit you perfectly; I mean which do you look at and just feel it fits with your mental approach - and yes, that makes more sense when you're looking at them than it does when you write it down
). But 177's right - nothing wrong with a second-hand 60x series at all, it's how I started (a DURC club FWB 601, which I shot for two years or so before getting my own).