I have no experience with the 300T, but I know it has a 1200 ft depth rating (980 ft for the 300) it has an Helium valve and its movement that is not COSC certified (the 300 movement is certified and has not He valve).
The Doxa Army has the same rating as the 300 (980ft) and its movement is not certified but it has the Helium valve as the 300T.
The 300T has a lower retail price than the 300, probably due to some difference in the movement which makes the proce difference. My Doxa Army is accurate as the 300 in spite of not being COSC certified.
Hope these comments help to answer you.
Best cordial regards, Abel