Like Moses said above, the MM300 in on par with a Sub but the GS is at another level.
Whilst the GS is clearly the better watch in every objective way, the MM300 has a charm I've not felt from another watch. It's not possible to understand the appeal of the MM300 without having worn it for a while. They are not as easy to live with as a Sub, but they are not easy to live without either and commonly those that sell them end up missing them. I would say without reservation that in the flesh it is one of the most attractive dive watches I have ever seen.
An intriguing watch and absolutely worth trying out.