I spent 7 years banging SPs. I can tell you, with certainty, that if the MP industry goes to full menu, less attractive girls will replace the better looking girls that no longer want to fuck and suck every guy that walks through the door. Sps are generally less attractive than MPAs and slightly less attractive than your typical mid 20s civie girl. So, it makes sense that a less attractive girl would gravitate toward the SP world and have to give more to the client, whereas the more attractive girls would gravitate toward the MPA world and not have to give as much to the client.
If you want beauty, stick to the MPA world. If you want full menu, stick to the SP world. But be warned, SPs might disappoint you in terms of beauty. Very few attractive SPs out there. But, they are absolute sluts, so what they lack in beauty, they make up for in menu options. An SP is like driving a rusted-out beater that has a high HP engine swap done. An MPA is like driving a brand new BMW that isn't as fast as the beater with the engine swap, but is much nice to look at and will give you fewer problems during the drive.