Paul Mirencoff at Power Line has this comparison of the vulnerbilities of Rubio and Cruz. You already know about Rubio's.

Rubio’s vulnerability on immigration is reflected by the fact of his collaboration, as a co-sponsor of the Gang of Eight legislation, with the likes of Chuck Schumer and Dick Durbin. President Obama was eager, to the say the least, to sign Rubio’s bill.

In the case of the USA Freedom Act, Cruz was a co-sponsor along with Schumer, Durbin, and the likes of Al Franken, Patty Murray, and Barbara Boxer. President Obama was also a big fan of the USA Freedom Act. After it passed, he tweeted, “Glad the Senate finally passed the USA Freedom Act. It protects civil liberties and our national security. I’ll sign it as soon as I get it.”

(Read the Power Line article, here).