Texas is indeed the biggest prize of the conference expansion game, if under some circumstance the Big 12 went kaput. Let's cover some bases here with the most visible target of them all.
Texas state politics are a huge factor.
When the SWC dissolved, four schools from the state of Texas joined up with the Big 8 to make the Big 12. They were Texas, Texas A&M, Texas Tech, and Baylor. The other four SWC schools (SMU, TCU, Rice, and Houston) were left out.
Things could have been much different though. The Big 8 likely only wanted Texas and Texas A&M. However, Texas Tech and Baylor had friends in the Texas State Legislature, and they got to come along too.
Now, I don't know what the specifics are in the TSL now, but it's unlikely Texas is going anywhere alone. Texas leaving the Big 12 would cripple the conference. Because Texas A&M (at the very least) still has plenty of friends in high places, Texas won't be allowed to go anywhere without a plan for the Aggies as well. Those two are likely a package deal unless, say, Texas goes to the Pac-10 while A&M comes to the SEC.
Texas isn't really interested in the SEC.
You could take the word for Peter Bean, SB Nation's Mr. Longhorns. That's mostly enough for me.
Or, you could look at history. When the days of the SWC were coming to a close, Texas wanted no part of the SEC. Texas A&M was quite interested, but the guys in Austin were not at all because of academics. Only two SEC schools are in the Association of American Universities (Florida and Vanderbilt) as opposed to five of the old Big 8 (Iowa State, Colorado, Kansas, Missouri, and Nebraska), and the SEC was unsurprisingly not interested in raising admission requirements. Nothing much has changed since.
Yes, academics do matter.
You know what brings a university more money than athletics? Research. Texas, by all accounts, is very interested in moving up the research money rankings.
Let's take a look at the 2007 data on university research expenditures. Texas ranks 32nd nationally, which places it third in the Big 12 behind Texas A&M (22) and Colorado (24). For Texas to rise in research funding, it would be in the school's best interest to join up with a conference of other research powerhouses. Those kinds of associations matter.
In that light, the SEC doesn't look good. Only one SEC school ranks ahead of UT-Austin (Florida, 17th) and just two more are in the same neighborhood (Vanderbilt, 36th; LSU, 40th). By comparison, the Big Ten has six schools ahead of Texas (Wisconsin, 3rd; Michigan, 5th; Ohio State, 9th; Penn State, 11th; Minnesota, 14th; Illinois, 28th) and two more right there with it (Northwestern, 33rd; Purdue, 35th). The Pac-10 also has six schools ahead of Texas (UCLA, 4th; Washington, 8th; Stanford, 10th; Cal, 20th; Arizona, 23rd; USC, 25th).
That's just more evidence to the notion that if Texas is leaving the Big 12, it would more likely be to the Big Ten or Pac-10 than the SEC.
If it was to leave the Big 12, Texas' first choice is likely the Pac-10.
Let's go back to history again. Before the SWC went away for good, Texas actually went to the Pac-10 first, not the Big 8. Distance and time zones were a drawback, but ultimately objections from Stanford kept the Longhorns out.
I'd be interested to know if Stanford is still as resistant now 20 years later, since that would be one of two big hurdles to getting UT-Austin into the Pac-10. The other would likely be acceptance of Texas A&M, and best as I can tell, there is no information available as to whether the Aggies would be welcome or not. TAMU's large research budget and recent admittance into the AAU certainly would be in its favor.
The Big Ten isn't far-fetched, but it's not likely either.
After being rejected by the Pac-10, Texas reached out to the Big Ten. That conference had put a four-year moratorium on expansion after bringing in Penn State in 1990 though, and despite being intrigued by the possibilities, the Big Ten also said no to the 'Horns.
Of course, the Big Ten was probably a little skittish at the time about expansion since it didn't know yet how the addition of Penn State would turn out. Clearly that's not a problem anymore. It still may not be likely that Texas will end up in the Big Ten, but it's not as outlandish a thought as some might think.
The best thing for Texas is probably keeping the Big 12 intact.
The Pac-10 might be a great place for Texas in regards to academics, but it's terrible from an athletics standpoint. It would be two time zones away from most of the teams, and the school's national exposure would go down as a result of having many late night (for the east coast, anyway) road games.
The Big Ten would be a great place academically too, but making it work could be tough. Distance is an issue, but at least it's the right direction for TV coverage. There also is the issue of finding a place in a league where Ohio State and Michigan rule the roost, something that would be even more difficult to deal with than its initial troubles with Nebraska at the beginning of the Big 12's existence.
The SEC would be a step down academically for Texas. It also provides even more egos to deal with. Instead of just fighting with two schools, it'd have to carve out a place among the SEC's six power schools of Alabama, Auburn, Florida, Georgia, LSU, and Tennessee. Texas' stature as an elite team would be diluted to a degree by being around so many other alpha dogs.
If the Big 12 can get through this round of expansion without losing Nebraska, then I'd bet Texas would do whatever it can to keep the gang together. Utah (70th nationally in research, tier 1 standing) would be a great candidate for replacing a team that might leave, like Missouri (76th nationally in research, tier 1 standing) if it went to the Big Ten.
Being the kingpin of a major conference is a good thing, and the Big 12's TV contract woes can be solved with either new negotiations, a Big 12 Network, or both. I don't have any sources anywhere, and sometimes logic is not the overriding factor with these things. Still, I'd bet that if Texas had its way, its future would be in the Big 12.