Way back in 2006, I considered in this post the distinction between guaranty and guarantee used as a noun.
That analysis still applies, and it’s reflected in MSCD. To summarize, the noun guarantee is used to express a meaning analogous to warranty. By contrast, when referring to a promise to pay the debt of another, guaranty is the more popular form.
But one can find many examples of guarantee used to convey the latter meaning. It seems pedantic to insist on retaining guaranty for the latter meaning, in the face of use of guarantee for the verb and use of the alternative noun form guarantee.
I noticed today that Howard Darmstadter, Hereof, Thereof, and Everywhereof: A Contrarian Guide to Legal Drafting 201 (2d ed. 2008) says, “‘Guaranty’ may be the rarer bird these days, but it has the more precise legal sense that we are concerned with [namely a promise to answer for the debt of another] and should be preferred.” But for purposes of contract drafting, it’s a pointless distinction, because the instrument itself makes it clear that you’re dealing with a promise to answer for the debt of another.
Ultimately, it doesn’t matter what word you use. MSCD says, “So in all circumstances use guarantee as a verb and noun,” but that’s just me trying to promote simplicity and consistency.