06-16-08: TvadThey are a little different, but not in a meaningful audiophile way. The M5G arm has a little knurled screw in the gimbal to help keep the stylus in the groove when scratching. In fact, you have to remove it when adding the fluid damper.
KAB Technics product page mentions the only difference in the MG5 is better wiring in the tonearm and interconnects. I don't believe the tonearms themselves are any different.
I've read more than one review of a stock SL1200 that criticized its dark, closed-in presentation. I never experienced that with my M5G, which has always sounded extended and open, so my initial impression is that the better tonearm wire makes a significant difference, whether you get an aftermarket rewire of a lower-priced model or start with an M5G.