Personally, I hate plastic crates. I understand their purpose for airline travel and the like, but I won't ever buy one for regular use. Some people use them and think they're great, but not for me.
I (and most dog people I know who crate in the car) use appropriately sized wire crates. They're super convenient, easy to use and fold up nicely if you need to store them or transport them. Due to the setup in my truck plastic crates aren't really an option for me as I need the door to be on the crate's long side. Virtually all plastic crates have one door on the short side.
Soft sided crates are great if you can be nearby to supervise the dog, but I wouldn't use one if I intended to leave my dog alone for any period of time. A wire or plastic crate can pretty effectively keep a dog confined if it tries to escape for some reason, but I have yet to see a soft sided crate that doesn't look easy to chew through. As such, I wouldn't use one even with my dogs that crate well if I wasn't going to be with them the whole time. I DO intend to get a lightweight one that I can easily set up ringside at shows, but at that point I'm there with the dog and will still use my wire crates for crating the rest of the day.