If I were choosing just based on diving, not value, I would go for most of the time on Catalina. It's a barren rock so the visibility tends to be good to excellent even from shore. Plus, as noted, it's marginally warmer, those few degrees do make a difference. If you're willing to pay for boats you could see a huge variety of sites. I have always gone with the
Scuba Cat, great crew, stable boat. We've stayed at the
Seaport Village Inn, they are great at handling divers, including pickups and dropoffs at the Casino Point dive park. Great staff, pretty good breakfast, outdoor shower and rinse tub, and they don't mind if you drape your suits over the balconies. Don't be fooled by the pictures, the place is a motel with significant deferred maintenance. Some of the Catalina hotels are really fancy, though, and get upset if you have dive gear, so scout around. The hiking on Catalina is also outstanding, and the restaurants (were) pretty good. Jack's for breakfast, Lobster Trap for dinner. The Catalina Express ferry is a whole thing, make sure you get reservations for the Long Beach -> Avalon nonstop and allow plenty of time to get down the 405, one of the most congested stretches of road in the state, and I say that as someone who has been on pretty much every freeway in LA and SF at one time or another. They are pretty stable trimarans but you don't want to be spending more time in the channel than necessary. They do have extra allowances for divers' luggage, read the PDF on their site. You can even bring tanks if you want to, which you might, because steel tanks are not as universal in Catalina as you might think (I guess it's the name!). Also the fill/rental station at Casino Point has only 5-lb hard weights, no trim weights, so bring those if you really want them.
Monterey is a wonderful place to dive, viz is not as good but there is much greater variety of shore diving and all the amenities of a real tourist destination. If Point Lobos opens up I would spend one day there, you need an advance reservation but that's only an issue with availability on the weekends. (The hiking is also gorgeous.) The restaurants are outstanding, I like Vivolo's Chowder House for something casual, Passionfish for something fancy, but bad meals in Monterey/Carmel/Pacific Grove are rare if you do a speck of research. The wineries in Carmel Valley are hidden gems, I like Rancho Galante and Windy Oaks. The Cheese Shop in Carmel is one of the best cheese shops in the U.S. Obviously the 17 mile drive, Pebble Beach if you like golf, there's just a ton to do there if you get blown out, and the whole Highway 1 is worth seeing. Catalina and Monterey can both have crappy conditions, even in summer, just luck of the draw, so keep that in mind.
No matter which you choose, unless you have recent cold-water diving experience I would definitely get a DM for my first day. In Monterey I like Randall Spangler but I've had decent tours from all three shops, including one a couple weeks ago from the new Cannery Row Aquatics.
