ER or H&S. The Deltec shouldn't even be considerd. It uses a weaker pump than the other two, and it has a smaller reaction chamber. The APF600 would be the skimmer to look at, but its like a a gazillion bucks....
I have owned both an A150 and an RS135. The RS 135 didn't have the new impeller ER released, so I don't know how well it will perform now. I would assume better. I was very happy with this skimmer.
I also like the A150, it was a great skimmer. Brian at finsreef hooked up the CS. They both will do a great job. It just depends on your budget.
Its tough to listen to others speak about ER because next to nobody has used their new impellers. So take what everyone says about ER with a grain of salt....
HTH
Jim