Basically, I am going to share my honest experiences owning both watches. Best for me to put my flamesuit on.
Differences? There are many. From the movement to the technology. The MM300 is using an 8L35 movement which is basically an undecorated Grand Seiko movement whilst the Sumo is on the 6R15. The 8L35 runs on a higher beat as opposed to the 6R15 I believe. Another major difference is the case. The MM300 has a monocoque case which means it is one piece. Diver rating on the MM300 is also higher than the Sumo. From the movement and tech that are in both watches, it is clear the winner is the MM300.
In terms of looks, I also favour the MM300 more. There is a refined premium feeling about the MM300 which denotes a more expensive watch whilst the Sumo is more utilitarian and feels like a workhorse. In terms of comfort on the wrist, I think both are comfortable in their own right although there is a slight feel of solidity coming from the MM300.
In pure watch sense, the MM300 is the more superior watch in every way as compared to the Sumo and is highly recommended.
Having said all this, the question is why did the MM300 left my collection but the Sumo remained? The answer to this comes from the last sentence in your second paragraph. Its our common perception on the 'value' of the watch. I felt the same way you did. I keep thinking that there is just too much money in that MM300 with similar functions to the Sumo that could be put to other uses or in another watch. The Sumo felt to me at that time to have the best value-for-money. So the MM300 was sold and the Sumo remained in my stable. Hence my choice between a Sumo or a MM300 would be the Sumo.
P/S: But if you feel that you can live with spending close to RM6,000 on the MM300, then I highly recommend the MM300. It is the superior watch to the Sumo.