Both sizes would be fine depending on what else you decide to use the sump for. If you want to have baffles or add a skimmer at some point, the 20 would give you more space. The total population of pods
depends on a few things other than just space such as nutrient levels, flow or even mechanical filtration and the type of skimmer you use, if any.
To get a boatload of pods
(if that's the main objective) I'd use the 20 gallon with a simple air driven skimmer and small return pump. An air driven skimmer will still remove lots of organics but they're gentle enough to allow copepods and mysid shrimp to actually swim inside it to feed (seen it). A small pump may be gentler on the population and do a little less harm to the ones getting sucked into the main tank. I couldn't say for sure though.