They do have a performance edge but an Alfa Spider 2000 seems to cost double what a 124 Spider costs. I honestly prefer the looks of the 124 as well. I haven't driven either of them (but I will tomorrow!) but from my armchair it's hard to imagine I'll buy the Alfa over the Fiat.
Full disclosure: I'm torn between buying a pristine 1980 Fiat 124 Spider for $5k or a ratty but complete 1978 Alfa Spider for $7k. Alfa pictured.