What size sump pump for my 45g setup?

[byee]

I'm in the process of setting up my 45g cube with a 25g cube for a sump. Probably utilize 18g of the sump.

The head height from sump to the returns is 4'.

What size pump of I need for my sump? Is the Mag 5 @ 310gph at 4' adequate or should I consider the Mag 7 @ 475gph?

Thanks!