I've always like vertical drops for derailleur bikes. This goes back to when I had to notch the horizontal drops on Beloved Spouse's little French frame, so the Hi-E skewer could hold the wheel in place (!). In those mid-70s days, I took verticals as a symbol of the ability of the frame builder to do quality alignment. The horizontals were pretty tolerant of differences in effective length of the left and right chainstays; verticals had to be jigged exactly or the wheel would run crooked. Remember, this is back when not all builders were as careful as the norm today.
harvey sachs
mcLean va