Definitely take a tent – there’s nothing worse than trying to change in a cramped cold car, trying to find your kit down the sides of the seats and the noise of slamming car doors is annoying to everyone else trying to sleep as well.
Biggest tent you’ve got, if there’s space to hang wet/used kit as well that’s a bonus. Bin bags for all old kit on one side of the tent, all the new/clean stuff on the other side so you can easily find everything at 3am.
Tent will be warmer than a car as well although there is always the danger that it’s less waterproof and more likely to blow away. 😉