I just bought a "new" old bike and then sold some junk that's exactly the price of a green Brooks to match the new bike, and I'm going to take the plunge. I was about to order from my shop when I did a search to find that the B-17 is a full 3cm wider than my current favorite saddle (on all my bikes), the Selle Italia Flite.

I know a Brooks is going to be different and take some getting used to, but should I really only go for the narrow version, which is still a cm wider?

It's possible that the Flite isn't really the ideal width, but just that it was my introduction to non-cheapo, non-padded saddles, but it's was enough that I haven't ridden another saddle on any bike for 2 or 3 years now. I also ride in a very forward, stretched out position typically, but this bike has more relaxed geometry, if that makes a big difference.