Trump will likely have a lot of people who don’t normally vote republican voting for him which is something Romney did not have. Romney was not a candidate that got people excited and Trump arguably is. I don’t see Hillary as being very exciting for democrats or getting as many people out to vote for her as Obama did.
At the same time, I believe you are under-estimating the stay-at-home Republican vote that would be attributable to a Trump nomination. As a veteran of the Goldwater campaign, I'm acutely sensitive to how little support -- in terms of money, on the ground action and actual votes -- an anti-establishment GOP candidate can receive.
This effect, too, will be in play...to some degree.
That's not to say the very same effects wouldn't be in play with a Cruz candidacy.
Thus, both Trump and Cruz are risky candidates. But I would a helluva lot rather take my chances with Trump or Cruz than with Bush or, say, Kasich.