I owned both th MM300 and the MM600. First one was the MM300 wich I really liked, doesn't feel like a big watch at all on the wrist (I have 18cm wrists). I found it to be a lovely watch except for the stem/crown wich didn't feel very solid. Accidentally I stumbled upon a MM600 at a very competitive price so I couldn't resist and bought it. The MM600 is larger but it doen't look too large on my wrist. Very light due to the titanium. I immediately liked it a lot, much more then the MM300 and I realised I wouldn't need the MM300 anymore. Stem/crown build on the MM600 is super solid. The movement is the best you can possibly get..the sweeping seconds hand is so smooth it makes you smile every time you look at it...simply fantastic what seiko has achieved with their springdrive. 72hours powerreserve wich you can accomplish by winding the watch in less then 10 seconds. Fantastic lume..Beautiful laquered dial (same as MM300)..I simply love it...it puts my rolex deepsea to shame (LOL)...I would say..try one out..
Best,
Guillermo