We have a large PetGear and a medium (I believe) Snoozer. Both of the pups travel well in them. I prefer how the Snoozer sits in the seat, but the dogs are actually attached to the seatbelt instead of just the fabric of the seat like the PetGear. I feel that it is safer. A lot of the time, I just use the straps and put a blanket in the back seat when we need more space (for trips when we need every square inch in the convertible).
Here are some pictures of the PetGear and how it straps into the car. The last picture is my mom's dog, Zoey and how she prefers to travel
She is strapped into the seatbelt, and she likes to travel laying on her back... lol