Greetings. Most people these days don't use wet-dry's anymore instead they remove the bio-balls and use the wet dry as a sump to hold equipement skimmer, heater ect...You can also replace the bio-balls with live rock but that transition should be done slowly and with time.
That being said, IMO
you should have the largest sump you can fit in your stand because that increases the total water volume of your system.
If space is an issue I think there are a number of hang on the back filters that include a protein skimmer I think they are manufactured by CPR. I can't vouch for their effectiveness or quality.
Also from past experience most sumps that come with a protein skimmer the skimmer is not the greatest at producing skimmate, but again that's only my experience and opinion.