I think I'm the only member here who's owned both of these.
You can see my thoughts on the SBGA029 here:
http://thirtyfivemill.com/2012/04/05...ve-200m-diver/
I've also owned 5 different MM300s, my latest being this one which I own now:
http://thirtyfivemill.com/?s=mm300
It's a bit tough to answer "best bang for the buck" on this one as they're not really that kind of a watch and the one costs 2 1/2 times what the other costs. Quality-wise, the MM300 is right up there but the finish on a GS costing 250% more is always going to be significantly better and more refined. The GS is a bigger watch, totally different bezel, hands, dial, movement, power reserve and a 22mm bracelet. TBH, it's really about which style of what you prefer. They are totally different beasts.
Apart from which one you prefer aesthetically and whether you prefer the Spring Drive or Auto movement, it comes down to whether you're after more of a tool diver or a dress diver. If the latter go for the SBGA029. Otherwise the MM300 is an outstanding watch.