If you intend on spending most of your time in Manhattan, avoid having a car as parking is a total rip-off (it was like 25-30USD per 8 hours when I was there last before 9/11, so it could be just outrageous now). My suggestion is to avoid having a car altogether if flying in or make sure whatever hotel you stay at has free parking, then you can leave it there while you explore the city on foot (which quite frankly is the only way to experience it).