word choice - When should I use "guarantee" over "guaranty" and vice versa?

When would I use guarantee instead of guaranty? The dictionary definitions seem pretty much the same. Excepting maybe the noun form of the word.

I have a real world example. A website I'm working on has a 30 day money back guarantee(y?). If you don't like your purchase, within the first 30 days, you can get your money back. Is that a guarantee or a guaranty? If I was going to make a little advertising picture for it, would I say "30 day money back guaranty" or "30 day money back guarantee"?

I am leaning towards guarantee.

Answer

Short answer: you can't do much harm in using guarantee every time.

Longer answer: the New Oxford American English presents “guaranty” as only slightly different from “guarantee” in meaning, and goes further to list “guarantee” as a variant spelling for “guaranty”. In addition to the information research in Fowler (quoted, for example, here), it seems safe to say that you can use “guarantee” all the time without fear.