The only advantage the Seicento Abarth/Schumacher has over the Cinquecento is the wider track, arguably bigger wheels (personally I'm not a fan of 14's they make the car look Donk-ish, weigh a crap load and tyre choice is limited and expensive) and wheel nuts instead of bolts.
Use the later brake drums and front hub converter on a Cinquecento and you have the best of both worlds.
Due to legislation the Seicento ended up with crap like power steering, ABS and other unnecessary ballast that makes the car feel vague and diluted to me. The Sporting charm is still there but they just don't make me smile when I think about driving them. They are basically a standard small modern car no better or worse than a 107, Ka or the others.
Cinquecento vs Seicento will go on forever but you can discount any post 2001-2002 car for that reason alone. More stuff = More stuff to go wrong and more expensive to put right.
Having just owned a Seicento Schumacher and Abarth (in the last month) I'm glad to have got rid and even though the front is still smashed in I have my Cinquecento back and am looking forward to getting the old stick back on the road more than the thought of getting another Seicento.