IMO
Well how is the combined water volume going to be so large? Very big sump? Frag Tank?
I sized this APF600 with a 120gal tank and sump in mind. That will be my future setup.
Also read up on the main differences between the AP701 and
AP851. I believe I was reading somewhere one is harder to dial in because of its size? Not sure I will read up on it more for you.
MaxxII you think like me

200.00$ isn't alot in the big picture when your dealing with 2 skimmers that are both costly.
ER
ER has since redesigned there skimmer line. I won't go into details but they borrowed some designs from another skimmer company. Has this improved performance? Yes, to some degree.
If you do decide to lean more towards ER then look into ASMs
IMO the Deltecs have a very good Water Volume rating.
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Do I think Deltec skimmers are better then ER's??
Well on customer care/Support Deltec destroys ER!!! I mean no comparison.
"WHY is Support so important when the odds of your skimmer breaking are so low?"
Listen to me!!!! Support is on the top of my list now. I have learned that ERs support is there Achilles heal of there whole company. Flat out there support sucks! And they know it.
When you buy a Deltec your buying 1 on 1 support with Doug from Deltec.
Do I think H&S and Deltec are the same?
Almost identical.
Performance wise?
I can only tell you on my tank the ER that I had "PRE Mod" would have days where no foam would form on the top of the cup. The ER was more noisy.
My Deltec has more skimmate in 1 day with NO FISH then my ER would pull in 2-3 days.
If your going to look at Deltecs look into H&S also.
If your looking into ERs look into ASMs