Can't comment on diving in La Ventana because I have never been there. I was in both La Paz and Cabo Pulmo mid-November when our liveaboard was diverted into the Sea of Cortes by a storm. Both were good so it depends on what you want to see. At La Paz, I enjoyed Los Islotes the best because of the sea lions, and this time the dive site was mobbed with a huge school of sardines, so it was fun to see and shoot the sardines making evasive moves while being hunted by snappers and other predatory fish, sea lions, and sea birds - cormorants, boobies and pelicans. I don't think that the sardines will be there forever so ask first, if this sounds appealing.
In Cabo Pulmo, we did a dive at El Vencedor, and saw at least 20 bull sharks, some up close, in addition to a lot of fish. That was a pleasant surprise since on 3 prior trips, all I had seen was one, and it was pretty distant. On the second dive, the operator found the big school of jacks for us out in the open, away from the reef as I had seen it on a prior trip. It is all sandy bottom down to about 50 ft. with a dense and expansive carpet of Garden eels. They move around tho so they may not be in this exact spot when you go. This is the biggest school of jacks I have ever seen - beats the one in the old days of Sipadan, and the one at Dirty Rock in Cocos. It spanned from surface to almost the bottom, and at least the size of the Baseball diamond at Fenway Park.
From a logistics standpoint, the road to La Paz is easier than the one to Cabo Pulmo.
