Only been in a taxi once in Ames. I was too drunk to remember the name but it was cheap and a real nice lady drove us. However, my friend puked in the cab and it was a $50 fine. Moral of the story don't puke in a cab.
I can't assume it'd make too much of a difference which company you choose, how expensive could it possibly be? The 2 I know of are Nighthawk Cab and Ames Taxi, never used either, just seen them around.