I haven't owned either of these but I have played them both.
For a fingerstyle the 000-15 is a little cramped to me with the 1 11/16 neck but it does have a nice bluesy sound to it and is very dark and woody. Nice for a lot of folk stuff.
The GC3 is more of a jack of all trades to me. It can do some nice blues sounding stuff but is also very sweet sounding for things like "Dust in the Wind" or "House of the Rising Sun" Definitely much more treble then the 000-15. Also the GC3 has the gloss top which is nice. The 000-15 is a satin finish all the way around.
My Favorite Guitars has some custom order 000-15SM guitars for sale. They are the new all Mahogany version with a few nice upgrades and they've had them customized to have the slothead and 1 3/4" nut. Only slightly more then the GC3.
http://www.myfavoriteguitars.com/pro...roductid=16164
So if it's GC3 vs 000-15 I'd go with the GC3. If you toss the 000-15SM in the mix, that'd be my choice.
-Jim