I find that sovs generally command a high premium over the spot price (incl those without numi value). I suggest doing a comparison of fleabay's completed listings to provide a reasonable guide to what the market is willing to pay. McDonalds guide to coins and banknotes also provides insight into which sovs generally command bullion price + premium (starting 10%), and those that have a greater numi value. Don't take too much notice of the numi valuations in the book though (these are at the high end). You can buy the book off flebay at a much cheaper price if you grab a previous year's publication (and much of the info is still relevant/the same). there are other books or websites that have the info though
I like sovs (and old gold coins from other countries) as they are easy to move on if I need to cash in, there always seems to be a demand and they tend to maintain a steady price. Mostly though, I enjoy having coins that have some history, and I will hunt out any with some numi value. Its not everyone's cup of tea though.
good luck with your stacking