Wow, this is a really old thread that's been resurrected, but I'll offer a few thoughts since it's been 7 years since my last post on it.
I would advise against any long-term vehicle mounting of consumer-grade instruments like a Davis or OS. It's really just throwing money away because they are just too flimsy to stand up to all of the highway driving and hail encounters. You'll break or wear out the bearings of the anemometer and wind vane every season if you chase regularly. The Inspeed is much more robust and will last longer, but it will still wear out with a lot of highway driving.
Instead I'd probably make something easily removable that could be mounted quickly at the start of a chase, that way you minimize the wear and tear.
If you can afford it, consider something with an ultrasonic anemometer that has no moving parts. That's the only thing I personally would consider doing myself if I was inclined to collect mobile data again.