i'd definitely recommend booking ahead of time. Use one of the many listed already or we used Ashton Pitt on this last trip. If you go through your hotel, they will probably charge more.
Your driver will be waiting for you when you arrive, will assist with your luggage, have your preferred beverages cold and ready to go if you want, and make any requested stops along the way. As stated before, many will loan you a phone, you just have to put down a deposit and put some minutes on it. You know they will do a good job because they want your business for the ride back.
You could probably negotiate a fair price on the spot, but you won't get some of the perks.