We just got a 14 GT500. SVT and Recaros are the only options. MSRP was $62,000. Dealer hit us for $1000 over. I've never driven a ZL1 but the Gt500 is bad ass. I owned a 2010 LS3.
Straight line... GT500 is faster.
Highway runs... GT500 is faster.
Track racing (auto cross)... Miata is faster around
Road course track... ZL1 is faster around.
The GT500 has piss poor brakes and too narrow tires to compete with the ZL1 on a road course. If you want a faster/quicker car in the street without modding, the GT500 is faster/quicker... marginally. They both make great power and you'll be happy either way.
Don't let the fan boy talk you down... A GT500 is not... just a mustang.