I've had both recently. I still have the UE but not the FS-1's.
The UE costs a little more, but not much($12-$15), since purchasing extra tips for the FS-1 is necessary. Other than the stock foams nothing is usable.
For SQ, the SF.3 has the FS-1 beat in every way except bass quantity and impact. Mids, treble, speed, soundstage, and detail are all better. If you can EQ bass with your source, then the UE can have some impact but will never be as meaty or full as the FS-1. If you like a quick, lively sound with more details the SF.3 is for you. If bass is first in importance for you then might want The FS-1.
Build quality is the same except that you can change cables on the UE's. The FS-1 is designed more to be worn without going over the ear and can be more microphonic. The FS-1 offers some more isolation with foam vs the UE with foam.
For the extra $15 you get a two warranty with UE vs. one for the Xtrememac , a better case, more tip choices, and replaceable cables.
I would suggest the UE, it is clearly a better phone. You can EQ and fix some of what the SF.3 is missing. The Xtrememac is a notch below in most respects.