We had 3 adults in a towncar, but cut it close with the luggage on the way home. we had 3 large bags, 2 medium, 3 backpacks, and 2 tote bags. It all fit, but if we had anything else, it wouldn't have.
The luggage and comfort situation really depends on how lightly you pack and how many souveniers you are bringing back with you.
When we had 4 adults, we took a van, which was only $10 more just so that we did not have to sit so close to one another on the trips to and from the airport. A half hour can be a really long time if you are super uncomfortable.