If it were me, I'd go with the 300-ext. Looks to have identical bioload limits, but it comes with a DC pump. It's going to be quiet (comparitively) and it has a greater water draw.
I'm presuming you're aiming for a heavy bioload with your tank goals?
I have been looking at these skimmers https://www.rk2.com/protein-skimmers.php
This is a rock-solid company and their stuff just works.
I would definitely go external to the sump if that is an viable option.
P.S. @PJ I used a wide range of ETSS skimmers with great success. I am surprised you found the skimmer to be properly made. AE Tech, the manufacturer, was local to me.
Be well and regards