The big policy differences seem to come from immigration (where Marco Rubio favors what amounts to amnesty and Rand Paul favors a more measured approach) and foreign policy (though Marco Rubio did vote for Rand Paul's bill to cut off advanced weaponry gifts to Egypt) where conservatives often come down on both sides. The biggest difference seems to come from style. Marco Rubio has clearly been listening to his establishment handlers, like Bill Kristol, and has decided to lay low during the recent battles with Obama. Where was he on the fiscal cliff or the debt ceiling? MIA mostly. And what bills did he sponsor? Mostly stuff you really don't care much about like S. 3471, to eliminate the tax on Olympic Gold Medals won by our athletes and the seminal S. 3570, the Vessel Incidental Discharge Act which is about as interesting as it sounds. His handlers did tell him to go after immigration, which is actually a relatively safe issue given that the mainstream media would be on his side on it. Conversely, Rand Paul has been at the tip of the spear on a variety of issues, from the fiscal cliff to the debt ceiling to aid to Egypt. He has sponsored legislation to audit the Federal Reserve and a National Right to Work Act. He has also constantly been in the media espousing conservative values regardless of what was the popular or "consensus" opinion.
If I had to guess, Marco Rubio's speech will start off about his modest beginnings and then talk in some broad strokes about how immigrants are great for this country and government isn't. That's pretty much how most of his speeches go. People keep saying how amazing they are but then he never really gives specifics about anything. There is really only so much of that one can take before you think someone is just a charlatan or has been talking too much to consultants (he's pretty impressive so I'm guess it's the latter). My guess is Rand Paul will give a much more fiery and specific speech about actual issues, putting our dire straits into perspective and how increasing freedom can help save us.
Anyway, it will be interesting to see how both do, both have 2016 at stake.