I used a borrowed 20 meter hamstick style antenna on a triple magnet mount set atop my Buick Rendezvous for a trip from Texas to Illinois in the spring of 2012. My rig was my FT-817 with a Heil headset/mic. With 5 watts then, I worked Canary Island, Honduras and around the U.S. It was not set up to be a DX dream nor did I QSO the entire trip. I threw it in the car to try it out and see how I liked HF mobile.
So my results are subjective, and I have nothing to compare to. It worked well for me, I was happy. I liked it enough to buy an FT-857D which I will use in my car and boat as well. I too am considering which antenna type to use now, and the Rendezvous has drawbacks for anything mounted on the bumper, the antenna being right next to the body for much of the antenna length. I was going to just buy some hamsticks or hamstick style antennas since they worked well enough and could be interchanged from car to boat and given what I have read so far on several sites, I don't see a reason not to do that. It's also at least $300 cheaper than a screwdriver antenna, and since I won't be mobile forever I don't see an immediate need to change bands on the fly. Rest/pit stops serves well enough to swap out a hamstick.
That's my current leanings anyway, but still looking at the info because I won't be mobiling for a month or two at best. This thread did leave me with the impression that there was a lot of discussion but not many answers to your question, so I thought I'd toss in my subjective review. "The best antenna is the one that works for you" or something like that!