Your antenna will be strongest opposite were you mount it if you mount anyplace other than center of roof. If you can hear a mobile rig that is not an alligator station you should be able to talk to them. When you are talking about base stations and beams and power above about 30-50 watts all bets are off! You need to understand why they cant hear you. Heck if a guy has his beams turned and his back door is to you and he is running 50-100 watts through a Laser 500 for example you will hear him just fine but he will not be able to hear you! It might not be ground propagation you are hearing either.
I have not tried the latest Firesticks but for 20 years I told them they needed a tuneable tip 5 footer. I suspect a 102 to 108 inch whip would work best and have the best combo of power handling, bandwidth, price, durability and real world radiation. My preference based on my experience is 102-108 inch whip, open external cap antenna, Wilson/K40 base loaded magnet mounts then Firestiks. That said it has been at least a decade since I last played with a Firestik.