Each skimmer was run on my tank for 1 week, one after the other. Similar feeding, no major changes over those period.
After the end of the week the volume of liquid skimmate was measured and all the slime was collected from around the collection cups. I'm not quite sure how exactly to compare the volume of proteins collected by each accurately. Probably need to dry the samples and then weigh them and figure out and subtract the weight of salt.
The MCE was adjusted to about the best I could get it and it always filled the collection cup with foam and then the nice dry foam overflows all over the place and makes a mess. (Yes, I collected all that spilled gunk) So it's not really possible to run the MCE any wetter. The Prizm has a MUCH bigger collection cup which didn't have that same problem. But here's what I have...
Results:
MCE 600
Liquid: 59cc
Total weight(Liquid+slime): 61g
Specific gravity: 1.028
Prizm
Liquid: 285 cc
Total weight(Liquid+slime): 279g
Specific gravity: 1.026
The Prizm's skimmate is noticably less concentrated, but there's almost 5 times as much of it. I tried taking 10cc of each and diluting the MCE's to about the same darkness and it took 20 ccs. So, the MCE would be about 3 times as concentrated but about 20.7% the volume. So, by that logic the MCE600 would have removed about 2/3rds as many proteins as the Prizm.
I think saying the Prizm skims better than the MCE600 is hard to say given that my comparison isn't very precise. But I think their performance is close enough that price becomes the major factor. If you don't have an unlimited budget you'd be better off with the cheaper Prizm and a couple good powerheads. And, realistically, if you're looking at a HOB you're probably trying to keep the costs down.