I've listened to both at length and prefer the F1 to the F2, but I'm a big fan of both. The F1 was always able to image more preciesly for me (better than virtually any other larger speaker I've heard). The tradeoff is it sounds more significantly different outside a relatively narrow sweet spot where the F2's mid seems to cast a wider net.
The F2 seemed more forgiving of being run with a lower current, less powerful amp. The cheap receivers I've tried out leave my F1's thin, with a vague image and nasty, siblant treble. The F2 obviously sounds much better with a good amplifier, but it doesn't punish you as badly for a poor one. Both seem to be equaly bad at 'glossing over' poor source material...the Klipsch way.
Bottom end was comparable for me out of both - the 15 didn't seem to add any range down low. Seems to me the 12 is capable of higher excursion, so the two might be matched in terms of overall displacement but would likely leave the larger driver better able to 'room match'. As I've mentioned before, I had to pull mine over a foot from the wall behind before the response evened out. Close to a wall, or in a corner, and I would have pockets of crushing bass other places in the house, and thin, bassless response where I wanted to listen.
The F2 had a magnetic grille, whcih I see as superior to the peg/rubber rig the F1 has. Finish on both was extremely nice, very high quality.
Long story short, I kept the one I liked better but they're both a hell of a speaker. Give em some juice, spend some time tinkering with the placement and you won't find much better, anywhere. My opinion anyway - my favorite Klipsch.