I have experience with the BPS and it's original predecessor the Remington 17. Both in 20ga. I have been pondering the new Ithaca ultra lights. It is a wonderful design for the lefty.
I'm on my second 20ga which currently has a low round count. I may be loading up 1oz of bismuth 5s to spank,some ducks this year.
I believe the 16ga BPS is built on the 12ga frame and the 28ga is built on the 20ga frame.
I'm a pretty big fan of the 16ga in general, but prefer the older guns that are built on a 16ga frame.