This is indeed an Interesting thread . I have been collecting Dive Watches since the Mid 80`s and have been very fortunate to have owned and still own many great Divers . I cannot talk about the Halios having never owned or seen one in person but I have had several MM300`s and currently own an SBDX003 Historical MM300 , I have owned 4 Stowa Prodivers and own both a DLC and a Bronze Aquadive BS100 .
The Seiko MM300 is a Fantastic watch with top quality Casework , phenomenal lume and an excellent movement . Great size and weight and very comfortable on the wrist .
The Stowa Prodiver is a very nice piece . Construction is very good , but casework etc is not on a par with the MM or Aquadive ( Mind you when I was buying Prodivers their price was considerably lower than the MM ) A Solid Movement,some fun Dial Colors and overall a very good bang for the buck .
The Aquadive`s have been a wonderful surprise to me . The build quality is exceptional and on a par with many watches that I have costing 2-3 times the price . The Cases, and Bezels in particular are very well done and I find the weight to be perfectly distributed on the Isofrane Straps ( I am a little biased in that I wear Isofrane straps on many watches and love them ) . Both of my Aquadive`s keep time within COSC Specs and in particular my Bronze Aquadive has become a watch I grab very often.
Very tough decision to pick just one . I would have a hard time picking between the Aquadive and the MM300 and feel fortunate to have both Good luck with your decision but I would say Aquadive or MM300 will both leave you very happy .
All the best,