Speed without a doubt. I would even go as far as saying I would wait for Top Thrill Dragster even if all it did was launch and go straight into the breaks. I live for it. Hence my involvement with the IHRA, NHRA, and my brothers Alcohol Dragster.
Gotta love the power, acceleration, and speed. If you ever get the chance to see a real life professional drag race, do not let it pass by. 0-335 mph in under 4.5 seconds isn't something you can put into words.
I was "nervous" about the airtime more than the drop when I first rode MF. It's still not my favorite ride element. I enjoy both loops and speed. The loops can make you a little disoriented, and give you a nice view, while the speed makes you think "I'm going to crash!"
If I had to choose, it would be speed. You can't get as sick just going fast, which allows for more re-rides. After 6 rounds on the raptor, (yes, I did have to go thru the line each time) I was told that I was changing color and should not ride again.