UT doesn't really have a stranglehold on the top TX recruits. Stoops is already pulling plenty of them to OU. A&M is third in-line for TX recruits behind UT and OU. Moving to the SEC in itself isn't going to change that.
Stoops is playing 4 teams from Texas every year. None of the SEC schools can make that claim. ATM to the SEC opens the state to SEC recruiting and Texas doesn't want that. The last thing either UT or ATM want is to open the state to recruiting from both Pac 10 and SEC schools. I think you will see them both in the Big 12 or both in the Pac 10.
Big 12 makes the most sense from a recruiting perspective for both.