if you`re diving to 50' then i`d definately recommend a dedicated freediving fin, pick the right one and the efficiency power and energy wise is amazing, i`d also recommend looking at both jb esclapez black fins and omer millennium competition. both are nice stiff, snappy and powerful blades, i`d personally go for the millennium comps, in my opinion omer have better foot pockets and the interchangable blade system is good if you want to change the stiffness without forking out for a while new set of fins. as for mares quattros, they are essentially a scuba fin, i have a pair, they are powerful but they don`t compare to longfins. if you want a fin that is inbetween, have a look at the imersion plongee, the blade is 10cm shorter than the esclapez black fins but the construction is the same. also, the blade length makes them less cumbersome if you are traveling with them and makes them more suited for scuba. hope this helps. i speak entirely from my experience, i made the decision to have separate fins for freediving (jb esclapez) and scuba (atomic splitfin) i use the quattros for underwater hockey. all work well for what i use them for, i don`t really use the longfins for scuba unless im diving in warm water and i definately never use the splitfins for freediving. i occasionally use the quattros for both. definately check out the omer millenniums, they come in a variety of stiffnesses, i personally prefer the competition blade. good luck