External antennas will give you better range on n66.
Ac56 router has more horsepower to handle havier lan traffic.
If you want both, you'll have to go with ac68 router.
http://www.smallnetbuilder.com/lanw...rk-knight/1226-asus-rtac56u/1235-asus-rtac68u
http://www.smallnetbuilder.com/lanw...rk-knight/1226-asus-rtac56u/1235-asus-rtac68u