I have a 300 on my 29 gal SH tank. It does the job for me, but I took the plastic cup out that creates the alternating flow, the noise created by the constant suction every 5 seconds got old real fast. Works great without it. I'd go with the 500 on a 37. They're not real special and sound more attractive on the write ups about them. Also they are a lot larger that I thought they were going to be on the back side of the tank. The best thing about them is the heater that can be placed inside of them instead of the tank. They have a telescoping siphon tube and two adjustable outputs are nice as well.
Dave