Personally, unless I was going to run it suppressed all the time, I would go with a pistol round, simply because the .223, BO, or 7.62x39 are obnoxiously loud and have terrible muzzle flash / blast out of pistol length barrels.
I've got 2 in 5.56, one 7.5 inch, one 11 inch, and 1 300 BO with a 10.5 inch barrel...none of them are 'pleasant' to shoot without a can, even wearing ear plugs and muffs. I couldn't imagine touching any of them off inside a closed environment.
The 9mm, 10.5 inch, while not hearing safe by any means, is a lot more enjoyable to shoot with just normal hearing protection.
