We're bringing our car seat, primarily just for the plane (we bought our son a seat). The buses won't have seat belts, so a car seat wouldn't do any good on them. Cabs, most likely, would not have car seats available, unless you were using a service like Tiffany Towncar, and told them in advance.
I'm just hoping there's room enough in the cabin to store the car seat and a stroller, but there's not too many other options I can think of.