You definitely want a sump to increase the volume of your tank. Also, you can put heaters, skimmer, carbon and abundance of live rock in there so it is as I would say a diesel *** filter. I run a closed loop with the powerheads. I like the best of both worlds. Mag 5, 7 whatever is fine. Some people dont like the Mags, Ive had a mag 5 on a closed loop for about 2 years now and have never had a problem with it. It always starts back up without problem after turning it off for whatever reason, very reliable. I would suggest a good amount of powerheads and a lower gph rating for the return/closed loop. The reason for this is improved and more efficient skimming, if you run the water through the sump too fast you want skim the proteins out as well. So grab a mag 7 but 5 may be better for skimming particularly if you use an in sump skimmer which is what i recommend. MJ9 are more efficient than 12's if thats of great importance with you, for me its not they wont heat the water much regardless and i dont care about pinching a few bucks a year on power costs. Go to Marine depot if you go with MJ's and look at a product called the hydor flow deflector, it rotates the stream coming out of the MJ so you get circulation in more areas and more random flow. If you want to run an SPS tank you are gonna need alot of MJ's and you wouldnt have an SPS tank youd have a MJ tank. SPS ---high flow---imo mj's wont do unless you have 5 or 6 and all mod'd. For a 60 tank i would suggest about 1800 gph-about 2400 gph, but for softies you may blast them out of the water unless you keep them under rockwork or something , but this goes back to experimentation on placement as i said before. I would highly suggest going tunze or something that gives alot of flow if you want to keep SPS SUCCESSFULLY. Definitley look into the closed loop with a SCWD,but even if you do decide to do this dont ditch the powerheads, go with both. Like isaid you could just run a sump with a drilled overflow(please drill it dont buy some crap overflow) and a nice return pump like a mag 5 or 7 or even higher if you want SPS, but not any higher than a Mag 11.
Let me know if you want any more of my thoughts.