Fitting a later type 500 rear beam and springs will improve the ride (that's why it was changed in the 500 for the 2010 model year). The springs and beam are a matched set and are designed to work together.
Early 500's are quite rough riding; I test drove one in 2009 on 15" alloys, and this was a deal breaker for me. On stock 155/80x13 tyres and wheels, the 1.2 Panda rides better.
The 500C got the modified rear beam & springs from launch. This was shortly before the modification reached the hardtop, which led some dealers to adopt a rather dubious practice back in the day. When prospective customers for a standard 500 requested a test drive, they'd take them out in the 500C, knowing the ride was significantly better.
You can't (well, shouldn't) fit the later type of 500 springs to a Panda or earlier 500 unless also changing the beam - you need to use the correct springs for the beam.
There's no meaningful weight difference between the 500 and the Panda; the exact numbers depend on the variant and options fitted, but from memory, kerb weight of a 100HP is slightly, though not significantly, greater than a 1.2 500 pop. I'd be interested to know if the 1.2 and 1.4 500's use the same rear springs. The only real issue in fitting a 500 beam & springs to a Panda is that it widens the rear track slightly. It all fits on the 'cooking' Pandas, but the factory fit wheel/tyre combination won't clear the arches if you put a 500 beam on a 100HP.
I've found the biggest factor influencing ride & handling is the weight distribution; with a heavy load in the rear (four passengers, or a couple of bags of sand in the boot), the change from being 1 or 2 up is quite noticeable. On those occasions when I'm carrying a particularly heavy load, I fold the rear seat & keep everything as far forward as possible.