1/8th will give you a more durable car. They are 4WD and very stable. Cheap if you go the right way.
AD2 runs great but can be a handful. Simply it is 2WD with alot of horsepower and is very light. Very cheap to run if you don't try to stuff too big of a motor in it.
I have raced both.
Go Hyper 8 ($420). Put your own radio in with a $109 OS RG motor. Don't go big! Spend time learning the car in a sportman class. Learn how to drive and setup the car for tire wear and such before you go for more race bred motors.
Sportworks makes a great RTR with a really good motor in it. Good radio to boot.
You will be able to go more places with the 1/8th.
My 2 cents,