Well, obviously you will know best as it is your tank. I just know Carey uses (2) 750's in her 29. I was using (3) 750's in my 54 and upgraded to (2) MP10's. In total I have about 3500 gph
in my tank and the fish and corals seem to love it!
Out of your (2) initial options I would at least go with the 425.