It seems to me that there's not much difference in meaning when the words are used as verbs. In the Merriam-Webster Dictionary, one synonym of the word Ensure is Guarantee.
Of course the word Guarantee can be used as a noun: this offer comes with a guarantee. The word Ensure is only a verb, and I cannot think of any noun form.
As to syntax, the normal use of the verbs would be followed by "that..." or a noun, not an infinitive. For example someone could ensure (or guarantee) that something will be delivered, or ensure (or guarantee) someone's safety. But I don't think it's usual to ensure (or guarantee) to do something.
Of course, someone could promise to do something. The verb Promise is often followed by an infinitive. And the word Promise can also be used as a noun, as when someone gives you his promise.