I am a season pass holder for San Antonio as well as bgw, my bgw will run out in a few months. Here are my two cents:
Your San Antonio pass will get you access to the parks, and preferred parking if available. While the discounts were for a short time based on the pass, it is now based on the park. I live in Fl and I got the discount for f/m at 20% for a bit before they updated it to %15 percent. You should get the percentage for platinum at any park you visit based on the home rate.
Other benefits are hit or miss, mostly miss. Christmas town is bgw exclusive, if you want that, you need a bgw pass. Bgt howloscream has a free preview that I’ve gotten into with bgw and swsa plats. Anything you should redeem at a kiosk or online is a no go, flash the pass type deals are all inclusive(ie free beer at SeaWorld in Jan, ride again etc.) I have not tried ride again at bgw.
Ultimately the best price option is probably half swsa half bgw, and use the free tix for Christmas town and some friends over the regular season. If you’re only buying one, you need to determine how much photo key, Christmas town, and the local offers matter. If all you want is access to all the parks with a decent f/m discount, San Antonio is a good option. I would even recommend getting just one in a family for the preferred parking. It’s nice to go on days like the 4th of July and not bother with the trams.
Hope this helps!