Do We Need 2 Car seats?

Robin
I am due at the end of April and it's time to start making decisions! Ack!
My husband works a normal 9-5 sort of job and I work afternoons/evenings. We aren't set on our childcare yet, but are looking to find a sitter since we only need a couple hours of coverage a day.
My question: do we need one car seat for each car or do you think that we could get one and switch it back and forth?
As a bonus question, do you prefer the infant seats or the convertible? I'm looking forward to wearing my baby a lot, so I'm not sure we really need the seat to be super portable...