I use a mag 5, even though people told me to move to a bigger pump here is my reasoning for staying with the mag 5;
i wanted the water to be in the sump a little longer so the skimmer (and in sump model) could get to more of it. ALSO wondering whats the reason for a sump if you don't want water in there? its nothing more then a fancy closed loop otherwise.
Depends on your tastes really and your system. Lots prefer a slower turnover rate. I prefer to max the overflow GPH so I can get all the crap out as fast as possible. Your skimmer throughput needs to be the same or relatively close to the overflow GPH in order to make a difference. Ex. overflow 600gph, skimmer should be at 600gph give or take 100gph IMO.