Depends on what you are comfortable with, and what kind of service you get. I have had service that truly deserved a 20-25% tip...our driver was wonderful. He was courteous, fun, and very talkative...gave us lots of little tidbits of info about the area and what kind of things we could do. He was right on time, both coming and going, and never let us touch a piece of luggage.
Compare the two times we had to go in search of our driver..both at MCO and then when it was time to leave the resort. Add to that the fact that the driver merely drove us back and forth. My dh had to do most of the luggage moving/storing in the trunk. There was no conversation either way. It was almost as if the driver resented the fact that he had to drive us back and forth. He got a less then desireable tip. Why did I tip at all? Because I wanted the driver to realize that I knew to tip. If I had given no tip at all he may have thought I just didn't know any better.
So...I like to tip both ways. So take whatever the 20% (if warrented) is and split it up, in halves. You will most likely get the same driver both ways, but there are times when you may have different drivers. You want each driver to get his/her fair share.