John Sides has the keys to 538 and has posted a nice piece suggesting that Republicans are more likely than not to nominate an ideological extremist for president. This analysis draws heavily on Cohen et al's The Party Decides and includes one of my favorite all time scatterplots:
There's evidence of this same trend in other countries and at the state level. Just think of the California Republican Party's backing of Schwarzenegger in the 2003 recall despite his being well to the left of most of the party regulars.
Anyway, if you're a betting person, the above history tends to point to a Perry candidacy rather than a Romney one.