Have to agree with personal preference. I recommend just having a look at different vehicle packs and models from the past months, and even watching a video of someone showcasing that certain vehicle pack to get a better understand of the pack itself.
If not, download and play around with a few.
Personally, I change my vehicles every few weeks or so, to keep it interesting. Though I am from Australia, it is nice to patrol with vehicles from states around America.
It's up to you, is what I am saying. What you like.