performance - mk2 16v standard , mk4 1.8T if your tuning.
handling - cheetah V elephant , mk2 everyday allday.
Livability - mk2's are old so need attention , mk4's have issues all of their own.
Fun - mk2's are edge of your seat fun , no doubt a well set up mk4 can be fun
reliability - see livability
modding - no shortage of ideas and parts on either front.
mk4's are fat and dull to look at , they're even worse than mk3's.