I have spent many vacations on SPI and Mustang Island (MI). Port Aransas is at the north end of Mustang Island and the south end is Corpus Christi.
We basically prefer Mustang Island, but do not go to Port A. There are no decent restaurants in Port A and really nothing to do. We tend to do a couple of day trips to Corpus Christi from our condo on Mustang Island. Examples: Aircraft carrier, aquarium, mini-golf, horse rental, movies, restaurants, shopping. We cook most of our own meals in the condo. H.E.B. and IGA grocery stores are easy to reach.
While the SPI beach is "better", it is only marginally better. We go to Mustang Island because the beach is not crowded and we can get a condo with pool right on the beach. Sand is superb for sand castle building. It is silty with a hint of oil, so it really sticks together and castles over 6 feet high are easy to build. The beach crew keeps the beach free of debris and washed up seaweed. The water is warm and shallow.
I think the waters at both SPI and MI are similar: pee warm, small stingrays, and jellyfish. Not much in the way of waves. So very relaxing.
If you like to read, sleep, walk, jog, and relax in hot sweaty outdoor weather, then MI will be perfect for you. Tell me what you like to do at the beach and I can comment further.
I'm not sure what a beach snob is. I used to live on the shore and have been to many beaches. I would think a beach snob would only go to a beach in Hawaii, but one cannot build sand castles there.
And whoever said there are restaurants in Port A, well, I would question their sanity. You can buy shrimp right off the boat though.