I've been on Celebrity many times in both Retreat and Verandah cabins. Also been on MSC in the Aurea 'experience' and recently Yacht Club on the Meraviglia for a B2B.
While I love the smaller Celebrity ships (Summit etc.) I also loved the Yacht Club offerings a great deal. And MSC is now so much less expensive for Yacht Club than Retreat on Celebrity. For me, it's currently no contest to book with MSC. The Yacht Club service and inclusions (including amazing embarkation) are simply amazing.
If and when Celebrity offers pricing that competes with MSC I'll be happy to return but not until then!
Happy cruising!