IMO, unless you are going to get a Bubble King or a LifeReef, then whatever you buy will not likely outperform the ASM. I still use some ASM and ERs because bubble plates, cones and all of that are just garbage and they don't work as well as some of the old stuff. New Sedra Pump is like $88 at Marine Depot and probably still your best bet.
There are better skimmers than a ASM - there always have been... but they are usually in the $1000+ price range for needlewheels... or they are venturi, downdraft or beckett. ...these are not good for under-stand use.
I would not mess with skimmer pumps. The contact time, air intake and the whole package is what matters, not the pump (per se). If you are off on flow by 10-20%, then it might not work as well. I think that this is why some skimmers are better than others - the manufacturer just "got lucky" with a combo. For example, Reef Octopus had a line out for a while with Sicce PSK pumps. These skimmers were awesome and I will never sell mine. However, the newer ones with the bubble blaster and other pumps do not perform as well.
