Main difference is tubing size: 23.8 (road bar standard) for the Promenade, and 22.2 (upright/mtb standard) diameter, meaning the Promenade would need to take bar end brake levers or find 23.8mm brake levers that would work. I've tried a similar Nitto Promenade (though wider at 48cm) which has 22.2mm and it appears to be similar in grip area as the Promenade, which is to say, there's not a lot of room to have shifters in a typical position. The Boscos are much roomier in grip area in overall width.
If you're looking for something narrower than Bosco, I'd look into the Nitto Promenade offerings (B602, B603, & B617), the GB Champs Elysees, or the VO upright offerings (Porteur, Belleville, Left Bank, Montmartre) in addition the Grand Bois Promenade, which is a very nice looking handlebar.
David
Chicago