My gf has the SGS and I have the O2X. The O2X is much better looking.
Generally, the SGS has a bit better software, it lags less, it doesnt come to a halt when installing apps, and it has gingerbread.
However, O2X is supposed to get GB soon (it should have already been here, but now they're saying its the end of November), but there are some great custom roms for it based on a leaked version of GB, and also custom kernels which overclock it all the way up to 1400 Mhz if you want (or keep it at stock speed)... they say its super-fast on the cutom roms (Cm7 or Togapigi). I havent flashed an custom one yet but I probably will tommorrow, and the stock GB is coming, we just dont know precisely when...
I personally think the O2X is the better option, it is more future-proof, as the SGS isnt really young anymore. However, the SGS will probably have better software support from Samsung, while O2X will have to rely on the developer community (which is luckily very good) because LG is terrible with software updates.