I would say yes all around. You could do a 15 gallon under neither. There is a 10g sump build but it looks so tight I wouldn't recommend it unless it's available an all you can fit. I was unhappy with my 10g and went to a 15 on my cichlid build.
With a 20g tank, a sump will just give you more water volume and allow you to get some of the equipment out of the smaller display. You could also put in a small refugium for pods
and macro alga.
This will also give you a spot for a protein skimmer be it in sump or hob
of the sump. Can you drill the tank? I wish I would have gotten a drilled 75g instead of my current one.