I'd shoot for ~350-500 gph at the tank for a 75. you have to look at a pump's 'curve' to see how it will do against pressure. some move lots of water at low backpressure for little electricity/heat, but fall off quickly once you try to push up through pipe/elbows. these are generally called 'circulation pumps'. other pumps power through backpressure without batting an eye, these are called 'pressure pumps', though they require significantly more electricity to do it. usually a circulation pump is perfectly fine for a return if all you care about is electricity and you can find one in the right size class.
50w will run you about $2.50 a month.
for a 75 my shortlist would be:
Externals----------------------------
Velocity T2 (640 gph pressure)
http://www.premiumaquatics.com/Merc...D&Product_Code=M3-PS2&Category_Code=DolphinEx
Pros- absolutely silent, and pretty small for an external pump. plenty of flow (to perhaps split to another tank or separate fuge, feed a recirc skimmer,...).
Cons- expensive, not as energy efficient as others (98w), heats water, a bit of a pita to take apart. If you have halides you will probably have to put fans in the hood to evap&cool the tank.
this is my favorite, but just because silence is by far the most important thing to me.
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Panworld 40PX (480 circulation)
http://www.premiumaquatics.com/Merc...D&Product_Code=PW-50PX&Category_Code=Panworld
pros- energy efficient (45w), doesn't heat water much, quiet (next size up has a loud fan), not too expensive.
cons- would like it to be just a bit more flow, but it's probably enough to get the job done.
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Panworld 50PX (590 Pressure)
http://www.premiumaquatics.com/Merc...D&Product_Code=PW-50PX&Category_Code=Panworld
Pros- moves more water than smaller panworld, less heat than the velocity.
Cons- uses 90w, is fan cooled so will make some noise, but probably not noticeable behind closed stand doors.
Submersibles--------------------------------------
Eheim 1260 (602 Circulation)
http://www.premiumaquatics.com/Merc...OD&Product_Code=EHEIM1260&Category_Code=Eheim
Pros- Easy to plumb (submersible), only 50w
Cons- submersibles tend to require more maintenance than externals, can be difficult to dampen noise (likes to vibrate whole sump).
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Oceanrunner 2500 (625 Circulation)
http://www.premiumaquatics.com/Merc...oduct_Code=AM-70002&Category_Code=Oceanrunner
Pro/Con similar to eheim. Eheim has 'better' name that's been around forever, but many people say oceanrunner is a better pump (quieter, much cheaper, better flow/watt- 38w).