They are same except for ROM and RAM:
WRT54G: 4MB ROM/16MB RAM
WRT54GS: 8MB ROM/32MB RAM
As atministrator suggested, with some 3rd party firmwares increasing in size and features the WRT54GS can't hurt. Both units support Afterburner so if you want to save a little bit of money buy a V2 or newer WRT54G.