Refugium

[Jaws]

Is it more advantageous to have a shallow refugium or a deep one? The reason I ask is I have two choices:

1. I can have a 55 tank, measuring 60" long, 18" wide and 12" high, which would fit over top of my 18.5" high 70G sump. I was going to split this tank into two. About 2/3 will be the refugium and the remaining 1/3 will be a quarantine tank.

or

2. I have about 20" by 18" to work with in the side cabinet and lots of height. I'm not sure what tank I could use for this space but i'm thinking like a 15 tall or an 18" nano. Remember, I have lots of height to work with so I can even go with an extra tall tank if it's more advantageous.

I've also decided to go with an upside down sandbed in the refugium based on an article I read by Dr. Charles Matthews in the annual edition of Marine Fish. Sounds pretty cool. Suggestions are encouraged.