I always try to answer the question, "how will I use this guitar differently from those I already have?" To me, just getting a slightly different tone isn't enough of an incentive/excuse to buy.

I have a 12-fret I use primarily for fingerstyle, a 14-fret primarily for singing and strumming, a resonator for open-G slide blues, and a cheapie for traveling/camping. To me, spending money on a guitar that serves the exact same purpose as another becomes akin to just "acquiring things," which I always endeavor to avoid -- although not always successfully!