I got a Koralia Evolution 750 for my 55gallon tank. I have it all the way on one side. I am also running an Eheim 2215 on the other. It might be too powerful, but I no longer have stuff accumulating on the bottom of my tank. Also, there was a skin on the surface of the water before adding the Koralia, but now it is gone.
I have the hydor 750 for my 55 gal with a Rena XP3 on the opposite side. It took some tweaking to get the flow direction correct because it is a little too powerful for my tank. All the long stem plants are almost horizontal and close to the surface of the substrate in places. Having the extra flow definitely helped, but I did not need as much as I have.