I think it depends on the situation. It is not yet clear if 150T to LEO is real (and even if it is, you're likely sharing it with a bunch of other customers who might not want to fly with another rocket motor unless it has gone through quite a bit of vetting and paperwork; Shuttle banned the use of solid rocket motor 3rd stages after the Challenger accident) and it is not yet clear what the cost for a typical customer is going to be. Rocket engines designed to start in a vacuum are more difficult and expensive to develop and test than engines designed to operate closer to STP.