The .221 is a very fine cartridge. It is a shortened .222 remington, but is still large enough to improve on the hornets velocity with a resulting increase in muzzle blast. The .222 was a very popular benchrest cartridge for years and the .221 is also rumored to be very accurate. My only experience with one was in helping a friend load and shoot a custom fast twist .221, we were trying to make a quiet extended range squirrel rifle by shooting 77 grain sierra matchkings. Our accuracy wasn't the greatest, but I think we were just moving those bullets too slow, even for a 7" twist. I personally love the .221 and think its a great round, if I was looking for quiet I would just load it down. However if your first concern is noise and you don't handload, I would get the hornet. Factory ammo is still more common around here in the hornet as well.
But if you handload or you want more velocity than a hornet, but not as much as a .222, I would look real hard at the .221. You would have a much greater choice of bullets and velocities with the .221.