I have a '75 F112 hog, a '79 F512 rosewood and a '96 JF30-12 maple.
Here's my opinion:
1975 F112 - spruce top, mahogany back & sides
Pros:
Love the neck profile
Lightest of the three
More pronounced mids than the other two
Great sustain
It has the coolness of a no-Bling but name guitar like a Martin D18
Love the mini Jumbo wasp waisted body shape
Easiest to hold & play (for me at least)
Listen to the F112 in a sing-along session
Cons:
Not particularly loud (compared to the other two)
1979 F512NT - spruce top, rosewood back & sides
Pros:
It's iconic, as a young 'un my first face to face sighting of a 12 string was of this guitar in the early 70s, owning one makes you feel as though you've arrived.
It's a beast, but looks like a beauty.
Huge sound. To my mind, this and the F412 is where it's at in terms of 12-string guitar.
Listen to to the Guild F512 in a confined rehearsal space
Listen to Ralph Towner's custom Rosewood Guild 12 in a recording studio
Listen to the Guild F412 in a stadium
Cons:
Bloody neck-heavy! (F512)
1996 JF30-12 - spruce top, maple back & sides
Pros:
Great value for money, one to get if you want a Guild Jumbo 12 at a reasonable price.
Westerly's last great 12.
Great string separation & definition.
Excellent volume and projection.
Better weight-balanced than either of the other two.
Check out this incredible 10 string "JF30-10" with scalloped fretboard, florentine cutaway and raised strings modified by luthier Michel Pellerin
Cons:
Not as much bass response as the F512
The iced tea sunburst finish on my JF30-12 proved not to be as durable as the finish on the other two, a small area of finish on the underside lower bout disappeared leaving bare wood, the other two exhibit the "cracked window" effect when "dinged".
However, like all newer guitars, the first cut is the deepest.
My 0.1 cent - YMMV.