The 500 Abarth is a remarkable little gokart considering its roots as a cheap economy car. Versatile, lots of fun, easy to live with. The C-model was even more enjoyable as it gives you a sort of 3/4 convertible feel, especially since the top can be moved at moderate speeds without stopping.
As Greg said, despite the FWD and weight distribution, the 500C Abarth is a joy on backroads and does a good job communicating with the driver. You can quickly find and exploit its steering and handling limits without much drama. It's very easy to control both understeer and oversteer with a bit of throttle. At higher speeds, the short wheelbase and tall stature made for some vulnerability to strong crosswinds or uneven highway surfaces, but nothing major.
Ultimately, I missed having RWD and a true convertible top. The 124 also won me over with better apparent build quality and interior materials. (I also fell in love with the Classica's low-tech, no-touchscreen, basic interior)
Interestingly, I feel like the 500s are better set up on delivery. My 124's steering and alignment was a bit vague from the factory. Really sharpened up with a good alignment and caster angle adjust (thankfully, that's part of the Mazda chassis)
Have you tried multiple trim levels? The 124's seats seem remarkably different among each trim despite being the same basic structure. Since the cars themselves are practically identical (LSD excepted), it's pretty cheap and easy to firm up a Lusso or Classica if you find they are a better fit.