I have a 204 and 20 prac. When I built the 204 finding brass wasn't an issue, at around $25 per hundred. Like Greg said it works best with stainless mags to get to 2.3" oal. It is an absolute pdog slayer and has laid out it's fair share over the years, but is showing it's age.
I just got the 20 prac last winter and it has claimed 1 coyote, which it laid out admirably with a 40 gr berger. Velocities are close though I don't think I push the prac as hard as my 204 loads.
In this day and age of hard to find commercial brass it's a no-brainer. 20 prac or tac all the way.
If you really want cool, skip the 20's and go 17/223. Since I built one in March the 20's don't see so much daylight. 20 gr zmax at 4100 fps+ is absolute and total devastation on small soft targets like blackbirds. And it could be pushed quite a bit harder.