The 204 clearly outperforms the 223, it has a slightly larger case capacity and bullet weight to bullet weight the .20 cals definitely have the BC advantage. Now, does that mean the 204 is a much better choice than the 223, not at all. I love my little CZ 527 Varmint 204 but I've got five different 223's and still only one 204.
As for the 17 Hornet VS 204 Ruger, IMO, you're comparing apples and oranges. In theory, no cartridge should really be anymore "accurate" than another but some will definitely have a wider sweet spot and if you reload, that's always a good thing.
The 17 Hornet will definitely have a longer barrel life than the 204 but that's not to say a 204 has a short barrel life so unless you are an extremely high volume shooter, as in thousands of rounds per year, both will last a long time.
The 204 has a significant advantage in the fact it shoots a heavier bullet faster than does the 17 Hornet so the natural elements will have less time to affect the bullet while it's in flight. The shorter the time of flight, the heavier the bullet, and higher the bullets ballistic coefficient, the easier it is to shoot a particular cartridge, more accurately.