Trump is shrewd and will have no shame when it comes to attacks, but his personal life is kind of a poop show as well, so in theory I think it's easier to fight back. Hillary is a flawed candidate to me. No matter what people say about Obama today, there is no denying his campaign, in juxtaposition to the administration of Bush, generated some excitement and optimism. Hillary, doesn't do that. Rubio could steal a chunk of that Hispanic vote, which would be kind of the ultimate irony that the party of old white dudes would need the help of a group they are not very fond of.
I think Rubio just because he could appeal to the Hispanics. But to do that he will have to show a willingness to support a program for a path to citizenship. He has done so in the past. With that can he get the support of the base?
It's the same as with Hillary though, who else is the base going to vote for at that point? Republicans, unlike Democrats, actually show up. No matter how much they dislike Rubio, he will still be better than Hillary and the same probably works for most Democrats in reverse. I'm not sure I would see a ton of difference between Rubio and Clinton other than Rubio might actually be dumb enough to just repeal the ACA in hopes of showing Republicans how big his balls are. I do think he would support the path to citizenship.
Whomever it is you can expect a wild swing to the center after the repub convention. Gotta play to the base now then to the independents. Unless of course the nominee is trump. I really think he thinks his schtick will carry him all the way. He will see a difference when the adults start to pay attention.
Hope springs eternal, as the old saying goes.