Dan360,
The 204 Ruger is the same length as the 222 magnum, but it has less body taper and a longer body and has a 30 degree shoulder in place of the 222 magnum's 23 degree shoulder. This gives the 22-204 about 5% more capacity over the 222 magnum (when measured below the neck) which equates to about 100 fps. In contrast the 22-204 has about 8.5% more capacity than the 223 which equates to an advantage of about 200 fps (at the same pressure). 200 fps makes a surprising amount of difference in trajectory and retained energy. The 22-204 is ideally suited to pushing the new 60 grain NBT (based on my experience with the similar capacity 5.6x50 magnum).
Note also that brass for the 222 magnum might not be available for too much longer. 22-204 case can be easily formed from 204 Ruger cases which will be available for a long time to come, and it is available from a wide range of manufacturers. Yes 222 magnum cases can be formed from 204 Ruger cases, but this is a little harder, and why not take advantage of the larger capacity 22-204? Here's to hoping that Ruger introduces the 22-204 as a factory chambering. Maybe they will base it on a longer case (like the 5.6x50 magnum) and call it a 224 Ruger. That would give it about 300 fps over the 223, enticing more shooters to upgrade.
Having said all that, the 223 will not be displaced from the AR-15 format because of OAL limitations. But at longer cartridge would do well in bolt actions.
