The M9A1 and the 92FS are close to identical except for the M9 having the rail and the 92FS doesn't. The biggest difference in these 2 is, the M9 has 2 dot sights (dot in the front, half moon in the back) while the 92FS has 3 dot sights, like your PX4. There are a few other minor differences, angles at dust cover, radius on backstrap, and markings.
I have the 90-two, in 9mm and am totally pleased with this purchase. Have had 100% reliability with this handgun. The rail on the 90-two comes with a rail cover that easily goes on or off. If the rail is not in use and you want to holster this gun, put the rail cover back on and this makes for snagless insertion and withdrawal from the holster.The 90-two has the 3 dot sights like your PX4 and 92FS. Having (3) pistols with the same type of sights will be to your advantage. I personally just liked the looks of the new 90-two vs the M9.
Take a look for yourself : www.beretta90two.com
I think you would be very pleased with the 90-two and this Beretta would make an excellent addition to your collection.