A note about both Casa Mission and La Mission.
Casa Mission was always our favorite place. We would typically go there on the last night of our trip and it was a great way to end the trip on a high note. The ambience was the best on the island, service was great, steak & lobster was great, all good. It's changed a lot in the last few years and now feels like a tourist trap to me. Last few times we've been there the service was mediocre at best, the food was average, the prices were high, and they play the stupid exchange rate game with the menu prices in dollars then they charge you in pesos at an inflated exchange rate. They also seem to cater to large groups more than individual diners so if you're a party of 2-4 that just walks in the door good luck getting served quickly. I have no patience for the cruise ship passenger tourist trap BS so we probably won't be back.
La Mission on the other hand- it's been around for years and for some reason we had never tried it. My wife and I wandered in there one day on our most recent trip in November and had a really great meal and overall experience. Prices were fair, no games, and best of all they offer a discount for divers when you show your C card. I think it was 15% the day we were there.