I've owned both. The X170 is a much thicker guitar, and has a very different feel and tone than a SFIII. I've owned both, and the X170 had a very different tone, though mine had the single-coil switch which added some nice options.
The X170 is much heavier than the SFIII. The SFIII is ridiculously light compared with just about any other Guild I've owned. My SFIII howled and screamed with even the slightest chance for feedback, while the X170 was very well behaved. The SFIII was more lively feeling to my hands, probably for the same reasons (thinner, no soundpost, etc.).
My X170 was comparable to my Gretsch SSLVO, but the Guild was heavier while the Gretsch had a more lively feel. The X170 felt like a heavy Guild Acoustic. I *loved* the neck on the X170, though.