Santorum doesn't really have the legal heft to be a Supreme Court nominee. He only practiced law for about 3 years before he was elected to Congress. I mean, it's not that I don't think he has good legal judgement, but it really helps to have a lot of experience with the specifics of the law before you get to that level.
As far as defeated Senators go, Jim Talent might be a bit more credible. He was once Richard Posner's law clerk, and taught as a Law Professor. But his resume is a little thin for the Supreme Court as well.
If he wants to pick a Senator, he should pick John Cornyn. Cornyn's got an excellent resume, long legal career, State Supreme Court, State Attorney General, and his personal relationships with so many Senators would give him a much better chance of being confirmed. It would also open up his Senate Seat, which would allow for Perry to appoint Harry Bonilla to his seat. Bonilla would not only make a good Senator, but he'd be a good messenger to win over hispanics.