We used the car seat adapter. I thought the bassinet was cute, but not very practical, as it's easier to move a sleeping baby in and out of the car in their infant car seat.
If you get the car seat adapter, make sure it's for the city select. We accidentally bought the other baby jogger car seat adapter (not realizing there were 2 different models). Not a biggie, we just exchanged it, but thought I'd mention it so you can save yourselves and extra trip to the store!