I play the G312 and F512 at least once a week. More like twice a week, actually. Took them both to Ninfa's last month when I played my first solo gig. My two cents worth on their performance:
For beginners, I was a bit disappointed with 'Boogie' when I first received her last year. I'd read all the hoopla and expected it to play itself right out of the box, I guess. I took 'Boogie' to the luthier for a setup, replaced the plastic saddle with bone (the nut was already bone; go figure on that one) and had a b-band installed for electronic capability. Still wasn't overwhelmed and she sat in the closet rarely played for a while as I found and immediately became infatuated with the G312 (Hanzz) I purchased at the Texas Vintage Guitar Show in Arlington last fall. I also took 'Hanzz' to the luthier soon after the purchase for a setup and some minor repairwork. The sound from that G312 is to die for...
Anyway, long story longer, I started experimenting with 'Boogie' more often as I started preparing for the 21 March gig. I played the F512 every night capoed (both are stepped down to 'D') for certain songs, then when my setlist came to playing a 12er in 'D' (uncapoed), I'd just reach over and replace the F512, with the unplugged G312 (rather than fine tuning which I've found, happens almost every time after putting on or removing a capo). Distinctly different performance too, because Hanzz has no onboard electronics.
Here's the verdict... Once I gave her some play time, that 512 opened up like crazy and now plays itself... What a wonderful, beautiful, hunk of Guild Rosewood/Spruce... It is a bit louder than the G312; I think the extra 1" actually makes a difference. Now so as not to get one jealous of the other, I play them equally as often as I can, and they'll both go to Ninfa's with me again on Saturday evening. How much playtime for 'Boogie' to open up? In this case, about 20 hours... I'm sure each new guitar is a bit different. I can only imagine how 'Boogie' will sound as she ages.
So which one of the two Rosewood Guilds wins? No help here, fellers, it's a tie...it's a tie...
Finally: To ensure fair and balanced testing, and after experimenting with other (12er) brands, over the past 18 months or so, I've recently trimmed my herd by selling two Martin twelves and a Taylor 12. One now sits in the closet awaiting eBay, my one remaining Taylor 355-12. Very pretty, mint condition contender but tinny in comparison... Taylor 12 anyone? 950.00. Guild 12ers? Priceless...