Strange Magic! And a Saddle Conundrum

How on Earth can one bicycle handle like a stately city bike one moment, and like a fast touring bike the next? I have no idea, but I have just experienced it.

At slow speeds, the Royal H. mixte rides like a vintage 3-speed: stable, easy, upright. So nicely it performed as a city bike, that I was almost reluctant to take it on a "real ride" - having gotten used to doing those kinds of rides on road bikes with drop bars. But once out of the city, the bicycle was magically transformed into... I don't even know what, because I haven't ridden a bike quite like it before!

This beauty handled the 40 mile ride through the hills of Concord and Lexington as if it were a casual cruise to the beach. We rode fast, but I didn't feel the speed. We climbed hills, but I didn't feel the strain of the climb.

When wrapped as I have them, the Porteur handlebars offer 3 distinct positions: upright when held at the edges, slightly leaned forward when held further down, and more aggressively leaned forward when held at the corners. And the bicycle responds to changes in hand positions very nicely. The Silver shifters installed onto Paul's Thumbies (more about this ordeal later!) are as easy to use as Sturmey Archer trigger shifters. And my positioning on the bike is so utterly perfect that I feel no pressure on my hands whether I am upright or leaned forward. The ride quality - namely its versatility - is strangely magical. I have never met a bicycle that can go from stately to aggressive at the drop of a hat before.

The mixte is not 100% finished yet, though pretty darn close: We still need to install the lights, and the front rack in these pictures is not the one that will ultimately be on it. But most importantly, I am not sure what to do about the saddle: Because my position alternates between upright and leaned over, it's hard to decide whether a more narrow saddle would be appropriate or a wider one, and whether springs or no springs. In the pictures, that is a Brooks B17 S borrowed from one of my roadbikes - but I am not certain that it feels right on this bike. What saddle do you have on your mixte, and how do you like it?