Matthew Yglesias at vox.com tells us why Rubio is scarier. Here are a few of the reasons.
[Rubio is ] offering a platform of economic ruin, multiple wars, and an attack on civil liberties that's nearly as vicious as anything Trump has proposed — even while wrapping it in an edgy, anxious, overreaction-prone approach to politics that heavily features big risky bets and huge, unpredictable changes in direction.
Rubio wants to increase spending but cannot even pay for his proposed tax cuts.
Rubio has proposed a tax cut that will reduce federal revenue by $6.8 trillion over 10 years. Numbers that large don't mean anything to people, so for comparison's sake let's say that if we entirely eliminated American military spending over that period we still couldn't quite pay for it.
Among other foreign policy blunders, he promises to scrap the Iran nuclear deal.
Rubio's approach to world affairs essentially repeats the "let's have it all and who cares if it adds up" mentality of his fiscal policy. His solution to every problem is to confront some foreign country more aggressively, with no regard to the idea of trade-offs or tensions between goals or limits to how much the United States can bite off at any particular time.
Muslims and immigrants? Be worried about your civil rights too.
... After the Paris attacks, Trump vowed to shut down mosques where radical preaching might be taking place; Rubio said that didn't go far enough and that a Rubio administration would be willing to stamp out Muslims' freedom of assembly wherever it might present itself. ...
Under pressure from Ted Cruz, Rubio is now promising to start deporting DREAMers as soon as he takes office. He's even turned a wink-nudge promise to bring back torture as an instrument of government policy into an applause line in debates and on the stump.
Far from being the cool-headed sort portrayed by the media, "Rubio is known to friends, allies, and advisers for a kind of incurable anxiousness — and an occasional propensity to panic in moments of crisis, both real and imagined."
More than any particular policy stance, what is perhaps most troubling about the ebbs and flows of Rubio's positioning is the larger picture they paint of a tendency toward systematic overreaction.