the 8' (not 10") whip should perform the best but also the most troublesome with it hitting on everything.
I would use a single whip on the driver's mirror (farther from trees at the side of the road). You will need to make sure the mirror arm is grounded well to the chassis. The longer this whip the better. I'd look at a K40 antenna instead of Firstick. Maybe a 5' or 6' if they have one that size.
The K40 has a lifetime warranty. My dad runs them on his 18 wheeler. He's snapped a few over the years on low hanging branches. He takes the stub to the CB shop and they give him a new antenna.