My experience only with both of those antennas:
The Firestik and FirestikII are decent antennas but need a good ground plane to tune up SWR.
The Wilson Flex (and the Firestik FireFly) are much more forgiving of poor ground plane and will tune up almost anywhere. The Wilson Flex does flex, but if it flexes too much/too often it will break the wire coiled around it and ruin the antenna.
Where are you going to put your antenna, on what truck? If you are going to put it on your roof or front fender, I'd recommend the FirestikII. If you are going to put it on a bumper or a rear-door hinge then I'd recommend the Firestik FireFly. If you need flexibility for trees or whatever, either use a stiff spring on the bottom or a Wilson Flex, but if you use the Flex then be prepared to test it frequently and prepare for the possibility of failure.