I have always done under and over for my sump. Really like the simply 2 chamber design that has worked for me on multiple tanks.
I am not a big believer in complex sumps due to the main factor of size and function. Refugium does work but it really needs to be appropriately sized. 1/2 of your tank size would make it worth while.
Also, I usually leave about an inch and a half in between the dividers to accommodate the flow I like to achieve for my turn over rate. Some believe that the flow through your sumo should match your skimmer intake rate but that has never made sense to me as my point of view is the more time the water passes through the sump, the more chances your mechanical filter and skimmer gets to pull the gunk out.
This leads me to 2 chambers in the sump design, one is for mechanical and skimmer filtration and the second is for the return pump. I know this works and works well but of course there are a million way in this hobby to achieve what you want in your tank. There isn't really right or wrong but just what works for you.
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