I would get the CS only because I like recirc skimmers better. You could always buy the RS and do the recirc mod on it yourself and save a couple hundred dollars. Of course you have to be willing to cut up the expensive piece of equipment you just bought. 8O
The first pulls water out of the sump while injecting air into it.
The second (recirculating) is directly fed water via the overflow and recirculates that water while injecting the air. The thought is that ALL of the water from the display is skimmed thus increasing skimmer efficiency.
In the first design, you may reskim already skimmed water because of the random water the pump picks up out of the sump.
So, on that note. You only want to run as many GPH through your sump as your skimmer can process. Most shoot for a 10x tank turnover. IMO in some cases even that is a little high.
