Neither. In their current forms, both have their shortcomings: for SLI it's a lack of compatibility with all games, for XFire it's the 1600x1200@60Hz cap, and both share the dubious feature of offering little improvement over just a single high-end card. Get a 7800GT (yes, GT not GTX) or wait for the reviews on ATi's X1800 line. A 7800GT is just as fast clock for clock as a GTX and much cheaper. I still have reservations about ATi being able to deliver a significantly faster part without driving up the power consumption and heat over nVidia's 7800 line. To me it seems like ATi is currently in the position nVidia was in when it released the Geforce 5800 'Dustbuster edition.' ATi's crossfire seems too little too late and from the reports about yield problems on their latest GPUs (they had to cut down to 16 pipelines instead of 20-32 because the samples just weren't having any of it) just don't inspire confidence in their upcoming products.
Well, I don't know in which benchmarks a GT is as fast as a GTX, because in pretty much all benchmarks I've seen, they were only as fast in low resolution, low or no AF and AA. Once you go 1600x1200, 4xAA, 16xAF, the difference looks to me a lot more like the difference between the 500 MHz vs 600 MHz on the memory.
Which also means that even overclocking the core might do a lot less than you think. It's good for "mine is bigger than yours" (the MHz number, of course) bragging on a board, but not quite match the real thing anyway.
So basically we can aggree that the GT offers better bang per buck, but the keywords are: "per buck". It's a smarter purchase, yes. But getting GTX for a lot less money, it isn't.