Went to both last year, Carlisle is awesome, but the nats has drag racing.I'm actually planning on going to both this year! Life's to short, time to live a little.
Carlisle has a better food. The best food I found is outside the gate at the bottom of the swap meet, there is a house there, that a local civic group sell hamburgers and hot dogs really reasonable.
Tip keep a cooler full of Gatorade in your car. Drink lots of liquids, normally it is hot.