Swings and roundabouts.
Cinquecento
Pros:
Plenty to choose from.
More recent and more modern
Wealth of knowledge
Very good parts availability
Nippy and economical
Fun to drive
Cons:
Cooling system issues if neglected
Can be a PITA to work on (you WILL have to at some point!)
Cramped engine bay
Sporting model invariably thrashed by young inexperience drivers.
Slight rust problems
More expensive than a Panda
Panda
Pros:
Very enthusiatic following
Simple to work on
Plenty of room in engine bay
Surprisingly nippy for its small engine capacity
Cheap to buy (4x4 excepted, though prices are on the rise for all models)
Very economical
Fun to drive
Cons:
Rust, rust and RUST. Good ones last well but neglected examples rust to bits.
Getting old now - hard to find good examples
Will need work unless owned by enthusiast
Brakes a bit poor on non servo models
Parts availability not as good as Cinq
Low value leads to many being neglected - many tatty examples out there
Those who don't know think you are weird for having one
Conclusion
Both have plenty of virtues to commend them. The Panda will need a bit more care and attention being older and it will be harder to find spares such as body panels and trim. The Cinq will cost more to buy but there are plenty of spares out there at reasonable prices, however cooling systems can be problematic unless maintained well.
Really it's down to whether you want an older and rarer car, or a more recent and slightly more common car. Only you can decide the final outcome