If you feel drawn to the longer one, order the longer one. At the least you will be able to try it out and send it back. For free.
It took at least 2 or 3 weeks to start breaking in my Brooks. (maybe 100 miles or so). But even the very first time I rode it, it was a huge relief! Yes, then it felt like sitting on a kitchen chair but it was a COMFY kitchen chair!
I think the sprung seats may be easier to break in. Or at least the springs make them more comfortable right out of the box.
Do buy the maintainence kit. You'll want the proofhide right away. Putting proofhide on the new leather takes away the slipperiness faster.
I just tapped my B67. Yup, still sounds and feels like cardboard. But the bits under my sit bones are softening and dip a little. When I sit on this seat it disappears under me and feels custom made. So many of my biking pains went away or eased up once I bought this seat! (even the numbness in my feet, which surprised me)
A b17s would look very nice on my "new" bike. I will talk to Bill once I get that bike and ask if he thinks it will fit me. (177mm wide. My sit bones are around 170-180mm, so that's pushing it) Whatever I end up doing (trading seats with the old bike, buying a new B67 or B17S) I doubt I'll ever buy any brand but Brooks again.
"If Americans want to live the American Dream, they should go to Denmark." - Richard Wilkinson