Any of those options are viable. Does that help???
First, lets get definitions straight.
A Deep Sandbed (DSB) is generally thought of as being 4-6"+ inches of fairly fine sand and is a great way to help with nitrates and will provide a home to many beneficial organisms.
A Shallow Sandbed (SSB) is 3" or less (usually 1-2") and although it won't help much with denitrification, can still be home to lots of little creatures that help to increase biodiversity in your system and are a valuable part of your clean up crew (CUC)
Although a DSB can be very useful for nitrate reduction, I don't care for them in the main display simply because of all the space they take up that can be better used as swimming room for the fish and/or more room for corals. I used a DSB in a refugium when I kept a 4' Eel that was a very messy eater. The DT was 150g, the refugium was 120g with 6"+ of sugar fine sand, and I also had a 50g sump. Prior to putting it online, nitrates were consistently untestable due to their being way out of range even after 50+ gallon water changes. Within a month or so after it's beginning, nitrates were nearly undetectable, even going as long a 2 months without a water change.
To be useful/effective, the DSB needs to be fairly large in comparison to the DT. IMHO, putting a DSB in a small section of the sump is just a waste of time and effort, and will more likely cause more trouble than it's worth.
A viable option is a "remote" DSB that can be as simple as a 5g bucket 2/3's full of sand that is fepd from a small power head or manifold and allowed to overflow back to the sump. The downside to that is that not many want a 5g bucket full of sand in their living room. If you have a basement sump or fishroom so that it and the sump can be away from the DT it would be just fine.
Another option is a bare bottom tank. Personally, I don't care for them because I just don't think they look right, but that is just my opinion. There are many who swear by them.
My current 120DT has, if it were spread evenly, about 3" of fine sand. I have allowed it to be pushed around by the powerheads so that now, 4 years in, there are spots that have an inch or less, and other areas where it approaches 4-5". My Leopard wrasses love the deep part, where they sleep every night. The shallow areas are all near the front so there is plenty of room for corals and clams.
hth!
