What I would say is "I don't know enough about Mormonism to be able to pronounce whether I believe it's a cult or not. Look at the Mormon belief system yourself and make your own decision. It's not my place to judge."
The only other thing I can say is that I would not vote against Romney because he's a Mormon per se. There are plenty of reasons to not vote for Romney, but him being a Mormon is not one of them, as far as I'm concerned.
Having said that, Mormonism does, in my humble opinion, have some cultish aspects, and in any event it seems to be a needless addendum to New Testament Christianity. It does give me some pause; just not enough to disqualify a candidate from being considered for President.
Islam, on the other hand...
Some ex-Mormons have alleged particular intrigues within the LDS church that might make Mitt particularly vulnerable to flip flopping.