I have owned a total of 4 Guild 12 strings, 3 of which I still own, and played one or 2 others. best bang for the buck is without question the Westerly F-112, and there is no shoratge of them either. I really like the Westerly G-312 as well, and you can find a reasonable deal on these, and the G-212 as well. But the G-312 is a little heavy if u play standing (never played a G-212). I would say the F-112 and F-212 probably sound similar, and the size is more about preferred dimensions/feel than drastically different tone (but ive never played an F-212). there is a nasty video of Bluesypicky dominating a 212 lets see if we can find it.
I did find the video, and it turns out his F-212 is from 1975, same year as my F-112, which is the best sounding one i have personally played. so at the end of the day, for the tone and the money I am recommending the 1975 F-112 and/or F-212 or as close to that as u can get. i do believe there were some chnages in 1976 but there have been holy grail reports from that generation as well (see other thread "F-112 questions").
Also I played Twocorgis' New Hartford F-512, and do concede the tone from that thing was everything it claims to be and more. i missed the boat on getting one at a good price though, and honestly, the guitar is just too big for me. my 1980 G-312 that was in the case its whole life, and its newish strings, are mellowing out a bit, and at the same time im teaching myself how to play it. every night it sounds and sings a little bit more like Twocorgis jewel (still about 50 times heavier, however), and I am so happy to experience his guitar for 30 seconds, just to know what mine should sound like and what I should be trying to make it sound like. Mine is heavier and a little louder, but the dread shape just works better for me. and it is gorgeous. i got lucky and mine has dot inlays (instead of none at all), and it was a fraction of the price of *any F-512, as well as any F-312...so don't sleep on those either, if it's RW your after.