I've been living in SD for about 2 years but I grew up in the TX hillcountry and San Marcos which is directly between Austin and San Antonio, soooo hopefully I can help ya out a bit!
PICK AUSTIN!!!!!!!!!!!!! Austin is soooooooo much like San Diego in regards to culture, attitudes, activities, nightlife, etc. Plus it's a great size city - it's smaller, BUT has a ton to offer...plus it's in the stages of growing and has more to offer everyday (though yes, traffic is a major problem in Travis County!...but they're working hard on that right now by building some major highways and toll roads...even a metro isn't too far away!) Austin's very diverse (for TX anyway), and politically if you're from Cali you'll probably avoiding pulling out your hair in Austin as opposed to other cities (Austin is known as our "hippie" city according to a vast majority of Texans - note the "Keep Austin Weird" shirts...). OH! Not to mention the LIVE MUSIC!!!! If you like it live, you will be in heaven. Plus there's 2 lakes, and the hillcountry with about 4 more within an hour's drive down some beautiful hillcountry roads. As well, the job market is better in Austin!! While it's more expensive than SA, it's worth it if you ask me. It's a classy city with a lot to offer - especially if you're young, single, etc.
SA is a nice city but it's, well, kinda drab if you ask me. I grew up going to SA allllll the time and it's fun and all, but it's far less "hip" and diverse....the diversity you'll find is within the Hispanic community...but that's it....the minority is a majority & it doesn't actually feel diverse or very cultural (Just because there's a large minority prescence doesn't make it diverse...that particular culture is predominant in the city...not one of many). However, there's a ton of history in SA and people are less likely to shoot you the bird for driving too slow. There's a lot of ghetto areas so that's something to be cautious of. Plus everything seems really spread out, so if you're into riding your bike, good luck. And SA always feels hotter than Austin even though they're really close...take that for what it is.
Speaking of hot....yes, you will fall over from heat exhaustion a couple of times during your first August in TX, but you'll adjust. The way God blessed TX with His hands was by providing a TON of lakes and rivers to play on - make sure to hit up the Guadalupe river and go tubing! It'll change your life

Plus, there's none of this "hmm, does it or doesn't it have AC and how far inland can I go without AC" business. Anywhere and everywhere you are there WILL be AC. And if you're not inside, you're playing on the water with 300spf on so who cares.
OH, and this business about TX not being pretty is obviously coming from someone who drove through west TX. Don't go there...it's truly a wretched place. Stay central though (check out Wimberley, Dripping Springs, Marble Falls, etc) and I guarantee you'll still be happy with the landscape even if it doesn't include mountains, valleys, cliffs, and gorgeous beaches. Dallas....eh. Don't go that far north...it's not all that pretty. (Plus from SA and Austin the beach is only 2-3 hours for a nice little weekend trip....though don't expect anything spectacular...though driving on the sand in a truck with a grill in the back certainly has it's own appeal)
Good luck!