For me seps before sub. Especially in a big car/SUV where there's lots of space inside the doors you can get a very good amount of bass. In some cases more than what you can get from those small underseat subs.

Another reason is the amount of space a sub needs in its enclosure to be able to work properly. Adding a proper sub will definitely take away some of your car's practicality, so might as well dip your toes and go seps first and see if you're satisfied before deciding to dive in with a sub if you're not.