I guess maybe it had partly to due with the manufacturer recomendation on maximum tank size, which I think we can agree is usually overstated based on the average hobbyist's bioload. But if we were to factor that in that as well (not just reaction chamber diameter and height and then there is contact time and I am sure other factors) and take their max recommendation and devide it by half, the ER CS6-2 would support a 125 where the CS6-3 would support a 150, not a large difference, but just something worth noting.
The MR-2 would support the same size system as the CS6-3, and with the MAG18 at $110 from MD, bringing to total to $360 as compared to the CS6-3 at $420 from MD, there is an intial $60. It may be not a great deal, but some people can live with additional monthly electricity but not the intial outlay.
I have no problems at all with the ER and think they are one of the best skimmers on the market. I guess we would all need to know a little more info to properly reply, such as tank size, height under stand if that is a variable, in-line vs. in-sump ,perspective bio-load, any pumps already posessed that may be capable to running the skimmer, which could make a huge difference. If your sump return pump was big enough, you could T it off and run your skimmer as well and not need an additional pump, again, we just don't know.
I don't think anyone can make a wrong decision with any of the skimmers listed, they are all quality ones, but there are so many factors, it is hard to rate any one skimmer over the other, which I tried to avoid. I was just trying to point out the advantages/disadvantages of each the poster listed. He/She was concern sbout the quality of the MR-2, which they shouldn't be as it is a quality product even though it is coming form an individual rather than a company.