I have the "3rd gen" R-300. Been using it for six years. Does the job. But I've never had any other so I don't really have any basis for comparison.
I have access to both sides of my sump so it's not a problem for me that the return chamber is behind the refugium chamber but would be a pain if I couldn't get to it from the back.
The return chamber is a pretty tight fit so I actually drilled a hole between it and the main (skimmer) chamber, installed a bulkhead and put my return pump in the main chamber, drawing water from the return chamber.
Another downside is that the area where water comes in from the overflow is inaccessible. It's a small area but it's impossible to clean.
I like having a refugium chamber and they did a nice job splitting the flow so that the water entering the fuge bypasses the filter sock.