too early to be exact. In my opinion, the wielder has the option to go from MS to EMS at will (or maybe only Sasuke can). I havn't really seen anyone with an EMS except Madara so it's unclear what all comes of having one. Since he has EMS, even if Sasuke activates MS, I can't see him going blind again. I would think that they're different levels in terms of abilities. He might unlock a new ability in EMS that he cannot do in MS state (he already can use his MS abilities in EMS state). There was no need to go a step further and activate EMS and MS was good enough for the moment.
It's like going Super Saiyan. Each level is more stronger but the wielder can choose which state to go in (using less stamina).