The BPS is heavier, built more robust, designed not to need the closer tolerances and hand fitting required in the Ithaca, and in my opinion much more durable as a result. A really great design if you like the features. The Ithaca handles great, but for anything other than Woodies in the close stuff it wouldn't be my choice for a duck gun, even if I liked bottom feed and eject, which I don't. I hate bottom feeding thru the magazine, but I had 3 BPSs and never a hint of a bobble. (and please spare me the tales of turning the gun over and moving the pump handle slightly forward to load straight to the chamber. Not hunting, not likely.) I shaved well over a pound off a 10 gauge BPS after doing a lot of study, and she never seemed to miss that metal.
I have seen a lot of Ithacas needing a gun doctor, but I have never seen a BPS that needed anything more than a good cleaning, which a lot of them don't get, because Browning recommends not taking them apart because they don't stake the feed latches in like Remington does, and they know the mechanically disinclined won't be able to get them back together.
A lot of people think the BPS feels like it has a longer reach to the fore end. If you measure it, it don't, but it felt like it to me too. But, I'm an engineer, not an ergonometrist.
A good friend has beat a BPS to death for 20 some years with nary a hint of an issue, but he does clean it after the mud baths.