Hmmm that's a good question Lex. When you parse it like that, it's tougher to answer, because it forces more specificity.
So, I'd say that if Rubio had switched parties early on, instead of caving on immigration, then he would have captured the Never-Hillary, Bernie enthusiasm. Sanders would have become a non-factor also ran, and Rubio would have straight-up Obama'd Hillary.
Now in the General, with the epic defeat of the great invincible Hillary in his sails, and all the goodwill he'd have from middle-class (white and blue collar) white guys for that, I think he would have had an easier time in the rust belt... especially with a "Hispanics love this country and we just want to work hard for the American Dream alongside you" narrative. Another thing he would have had going for him is a "first Hispanic" narrative, which I think would have been better for Hispanic turnout, which is typically abysmal, and has tons of room to grow. Another thing he would have done (like Obama) is created a clearer moral dilemma in voting against him. Putting aside party loyalty, it would have been a lot harder to justify outside of racial/ethnic prejudice. The bottom line is Rubio solves the likeability issue, and he's waaay too new to be susceptible to the "establishment" label. He's also way too new to have all the baked-in baggage that Hillary does.
So both... to answer your question. I think he wins both as a Democrat in 2016.
