I think they'll both do fine in a 20 gal tank. They both grow to about 3ft. Hoggies are CUTE but I'm more partial to colubrids. That Mexican Milk snake is a real beauty. I love those colors. I'd seriously consider keeping one but I like larger snakes. And even though they aren't venomous, they look like coral snakes
. I've also heard that some hoggies can be problem feeders and most kings/milks are pretty voracious eaters. As far as the ease of care colubrids are a breeze. Get a thermostat, set your heat mat or RHP to 86-90, throw in some aspen bedding, two hides, some fake plants and a water bowl and you're good to go. You can't lose with either option but all things considered, I'd go with the milk snake. You also become an instant member in the Kings of the Forum if you get the milk
. Best wishes with whatever you choose and please post pics!