Asphalt, no question. Sounds counter-intuitive, but it works. It keeps you honest with your form, which, if you keep it right, means no pushing off or skidding on landing - just light lift-and-set short strides. Grass can hide way too much until you are really solid, including undulations in the ground. I only run on grass I know very, very well, and I wouldn't recommend any grass runs until you're very comfortable with asphalt or concrete.