Just now saw your post. I have both the 500 gph and 1600 gph models with the controllers. The two 1600 are on a SPS frag tank and the two 500 are on a 40 gal breeder.
I would suggest the 1600 gph units as they have 3x the flow for hardly any additional investment. For a 50 gal, two should be about right.
JBJ customer service is great. I had a problem with a impeller shaft and they resolved it in 24 hrs after I notified them.