We have sailed YC1 i.e. balcony suite on Meraviglia and Preziosa, and a grand suite on Ovation.
For us the key things are Restaurant, Lounge and sun deck area and would rate as follows:
YC restaurant and coastal kitchen are equal in quality and service. On Meraviglia the restaurant is within the YC area, so it's only a couple of minutes from most cabins. Butchers' Cut on Meraviglia is superb, and the free Diamond (black) card menu there is perfect for us.
The lounge in all MSC Yacht Club areas is huge, and a true amenity, serving snacks and drinks from 6.00am until well beyond midnight....far superior to the limited RCI offering.
The Top Sail sundeck areas in the Yacht Club are extensive, with a drinks bar and grill food bar....breakfasts are just OK, not great, lunches are very good and they grill your meat/poultry/fish fresh to order. We love this area for lunch in the Caribbean. The Sundeck area has a small pool and hot-tub, which many seem to enjoy.
Some other thoughts:
There is a Yacht Club seating area in the theatre.
One of the YC privileges we enjoy is to be chaperoned to the front of any queue on return to ship.....there are butlers circulating with a YC paddle, and it's a wonderful perk returning from a hot caribbean port!
Space, privacy and excellent service in the YC in an extensive area which is a ship within a ship....but the rest of the ship is available when you want it, and then you can return to your private enclave!
I agree that RCI theatre entertainment is better, but would describe MSC as reasonable rather than terrible.....occasionally good, and they are trying to improve this in the Caribbean particularly.
Are you aware that, as a Diamond Card holder, you are entitled to 5% discount with MSC in addition to any other discount you get from a TA or on-line? MSC also have 'Voyager Club' discounts on select cruises, offering a further 5/10% discount and even a small number with 15%....all in addition to the guaranteed 5%.
To summarise, Yacht Club every time for us...without a doubt!