As others say, balance matters most (with reasonable portability, of course). Noise matters second most. Velocity is probably third, at least with a 7mm-08.
20-22" sure seems like the sweet spot. I've used a couple of 18-18.5" models and they were a bit too loud for me. I have a 20" barrel at the gunsmith right now, being cut to 16.5" and threaded for a can.
