I've ridden thousands of miles on the old GP4000 tyres, the new GP5000's tyres, and also the old and new version Pirelli P-Zero tyres. To be honest, I'd be hard pressed to tell the difference in a fully blind test, my N=1 experience is that...
- The Pirelli tyres in my opinion give a slightly nicer ride
- All the tyres felt fast, I doubt there is all that much real-world rolling resistance difference (EDIT: Apparently 4 watts difference, GP5000 is faster)
- The Pirelli tyres were easier to mount than the GP5000's (which seem tighter than the outgoing 4000's)
- Flat resistance and mileage seems slightly better on the Conti tyres
For what it's worth, I have always been a big Conti GP4000/5000 fan, but all my road bikes now roll on Pirelli tyres.