Rubio is more likely to win a higher level of financial backing. Plus, Marcos temperment and demeanor appear calm and steady compared to the bombastic and unpredictable Trump.
Cruz would work as well, but he has too much in common with Trump to present a strong contrast.
You do know that Cruz is leading the field in hard money donations and is #2 in soft money? Rubio isn’t even close. Why would that change all of a sudden?