Which is better: 1/2 or 3/4 Return?

Yeah it depends on what you want to do with the tank... if I was keeping SPS in a 50-70 pentagon, I would run about 2000+gph for tank circulation, thats ontop of a skimmer moving 1100gph in the sump. High flow, low pressure= nice current, reef surge zone, where SPS does best... crisp clean water, if I was setting it up with a surge tank i would do abit less circulation flow, about 1200-1800gph and then a surge moving 400-800gph. You can setup a 50 gallon moving 3000+gph and it wont blow anything over, it would be a nice smooth current, high flow, low pressure is king in most reef tanks