Hi,
I have the B17 Champion Special (the only 17 to get, IMHO) on one bike, and the Flyer on the other.
1) I am pretty heavy, and have to pedal smoothly with
the Flyer. I prob have a lousy stroke.
2) If you need the Flyer, go for it.I do most of my riding with it. If the big bumps are causing pain, then you may need it. The B17 does have some ablity to soak up a bump, seeing how you have such big tires (they are under 100 psi, aren't they? Half the idea of running a big tire is to be able to run a soft tire) that is likley all you need...I find overall the B17 to be slighly more comfy to sit on, maybe it's just the way it sits on the bike. Supposed to be the same top, but the subframe may affect the nose in some way.
3) If you have the handlebars below the saddle, go for something narrower. If they are the same, go for the 17 or the Flyer, if above
the saddle you just might want to look into the 67. Regrettably, this is just a rule of thumb, YMMV...
4) Brooks work like a hammock. They give, and the frame bends in and up to cushion. If the seatpost doesn't grab the saddle in the middle, it will affect the way the saddle works. In plain English, shove the saddle back (as often happens) and the nose of your Brooks gets as hard as a brick. Bricks are not comfy. If you shove the saddle back, you also need to deal with the setback issue. You can get seatposts with a little more setback, and there is a chuck triathletes use to get a lot more setback. It's not expensive, your LBS should be able to find it. I am sure someone here will remembe the name if you ask.
5) There is another option.... the B17 with titanium rails.