car rental

My husband and I will be traveling for the first time with our LO over thanks giving. She'll be 4 months and will need a car seat. Does anyone have experience with car rentals with car seats? should we bring our own? should I bring the base? is it easier to just rent everything? please help! this is our first trip as a family. we'll be flying to Portland, but also want to drive to Seattle with a car rental. Also, if anyone has taken the trip, is there an alternate way of commuting between Portland and Seattle that may be more convenient with a LO?