Don't know much about a .221 Fireball, but you can get some insight on the .223 in Handguns from at
http://www.handgunhunt.com/tech/t12/index.html
I would have a hard time deciding which is my favorite in Contenders for Varminting & Coyotes, the .223 Remington or the 6mm T/CU. I have used both with success, and like both equally well.
If I remember correctly (without looking it up) the various commercial reloads (Black Hills Shooters Supply, Ultra Max Ammo etc) loaded with 55gr. Bullets have averaged between 2800 to 2850 FPS out of a Super 14 .223 for the most part. In handloads I have achieved 3300 FPS (with certain loads) with 40gr. Nosler Ballistic Tips or Hornady V-MAX.
One of my favorite bullets is offered by Ultra Max Ammo in their remanufactured ammo line for the .223. The bullet in question is the 52gr. Speer Flat Base Hollow Point Hunting Bullet. I have shot a bunch of Prairie Dogs with this bullet and it is very devastating. A buddy uses this bullet for Coyotes with great success. I have shot the Ultra Max Ammo loaded with this bullet, but don't ever remember chronographing it. I do remember that it shot quite well, and the bullet performance was on par with my handloads with this bullet.
As to which one to chooce, that would be a tough choice. In terms of Performance the .223 is going to have an edge on the .221, but in terms of les recoil and muzzle blast / flash the .221 is going to have the advantage over the .223. On a different note most of the commercial reloaders use AA2200 Powder or something similar. This powder is known for lots of muzzle blast / flash in these small bore cartridges.
I am also quite sure there are more different brands and types of ammo available for the .223 if that is a concern. I guess in the end if it were me for Coyotes I would opt for the .223 as the added velocity / energy may be needed at times.
Larry