I'm a southpaw.
Back in the day, I carried a SIG P226 on duty. It was a marvelous weapon that never failed to do anything I asked of it. Only thing I did to it was swap the mag catch button around and mount night sights to it as it was private purchased off an approved list in lieu of an issued G17.
The decocker is right hand oriented, but as you noted it's mostly an administrative handling issue after charging the weapon. It does have to be trained for though for decocking it on the move during firing drills. I learned to use my right hand over the top prior to moving and slap the decocker down with the index finger, essentially copying the gross arm/hand movement that was required to sling shot the slide to RTB. Everyone must sling a SIG P series because the slide release button is uselessly small and inconveniently located to use while actually firing it.
The Beretta is a different cat. Again, the mag button can be reversed, but the slide mounted safety wings rather militate against slinging the slide overhand with the off-hand to RTB because you might accidentally decock the pistol and disconnect the trigger on an F model. (There is a school of thought out there that one can grasp the wings themselves and rip back and up, but that is another thread). To compensate, the PX-4 has a really generous slide release lever that is totally manipulable by the trigger finger.
I don't like the SIG P226 in .40 S&W. The pistol was not designed around this cartridge and its higher bore axis is not a help with the sharper recoiling .40 round.
My beef with the PX-4 SD is that it is not size efficient for the .45ACP. One would think that a polymer handgun would be able to flush mount ten rounds in the mag minimum, but it doesn't.
So, oddly enough, my breakdown would go like this:
9mm: Either the PX-4 or the SIG P226, can't go wrong, but you get 17 round mags with the Beretta.
.40 S&W: PX-4 hands down. The rotating barrel does a number on the recoil characteristics of the .40 round and once again, +2 on the capacity over the SIG.
.45ACP: SIG P220 hands down. Single stack round count with a single stack grip frame. Beretta? Single stack round count with a double stack grip feel.
EDIT: Another general benefit going to the PX-4's advantage is the wholly owned Beretta subsidiary M.D.S. which makes factory spec mags for Beretta pistols, but without the "P.B. Beretta" markup. They are regularly on sale on the internet for as little as $20.00 a magazine.
