If you get on a SpaceX ride share, you don't get to choose your own orbit. You'll have to make-do with an orbit that may (or may not) be close to your desired one. Unless you want your satellite to have propulsion, which might eat up any cost advantage. With Rocket Lab, you'll get the orbit you want typically, since you'll probably be the only vehicle on board.

I suppose if SpaceX had an enormous manifest of small sats they could bin them into similar orbits, but that would really depend on just how successful they are.