You can read quite a few threads on this topic!
It comes up regularly and I think was discussed just a cuple of weeks ago as I remember Bruce posting to the thread.
A soft crate will not give any protection at all, so x that off the list. Many are split between whether a secure, quality, tested seat belt harness (most are not!) is better than a good quality crate that is seatbelted into place.
Most breeders who regularly transport dogs would only ever use hard plastic crates, is what I have gathered from a couple of years on various lists. The crate should be large enough for the dog to stand and turn around and lie down but not be vast (so the dog can't be thrown around inside the crate) . I think a strong plastic crate *designed for transport* -- eg airline approved -- is the only type of crate that should be used. A flimsy plastic crate can be easily crushed. Some use metal crates especially when rans[orting lots of dogs for shows -- but most metal crates would collapse if hit -- so I have never used a metal crate for car journeys. If you have a van and can safely wedge in crates, or a 4x4, and transport a lot of dogs, then I'd use a metal crate maybe, but I don't think they are safe at all for collisions. Even if one bounced to the ground if thrown from a car, many would just pop apart (the fold down type most buy are hardly secured safely for an impact!).
Some of the harnesses sold have long leads on them that clip into the seatbelt slot and most of these are simply useless -- people leave the leads so long that the dog would slam into the back of the seat in front and would at best be flung about. So avoid those or make sure the lead bit is properly adjusted to a very short length.
Another thing to keep in mind is that if there's an impact and emergency services arive, many will not go near a dog that is NOT in a crate because a dog in pain will be biting and extremely hard to handle. Also some rescue services particularly in the US have a policy to NOT touch dogs unless they are in crates.
Also anyone transporting a dog should be aware of what seats in the vehicle have airbags -- as that is where a dog should not go! A small dog (like small children/babies) runs a significant risk of being killed by the impact of an airbag opening.
So that's a whole range of things to consider!