Are you shooting factor ammo or loading your own?
If you're shooting factory ammo not only is the bullet selections for the 204 extremely limited but just finding 204 Ruger ammo is extremely difficult.
Ballistically speaking some of the 20 cal bullets, when comparing them to equal weight 22 cal bullets, particularly tipped bullets like the Vmax, the 204 has the advantage.
But, when you compare the numbers with a bullet like the Berger 35gr, which a lot of coyote hunters use, to a 40gr 22 cal bullet that ballistic advantage the 204 had is suddenly nullified.
I like 20 cals and I've had a couple 204's in fact I just mounted a scope on my Tikka 204 last week but if I was going to pick between only having one or the other, I'll take the 223 every time. If you already have a good shooting 223 and you've got the itch for a new rifle go for it and get yourself one but I suggest finding some 204 ammo or if you reload some 204 brass first or you'll have a new gun and nothing to shoot in it because both 204 ammo and brass has been very hard to find for some time now.