I think STA has a lot to do with riding position. If you want a more upright position then slacker ST is better-- you're basically rotating everything backwards. Upright position also goes with more weight on your butt, squishier wider seat, and therefore you want your legs coming out forwards a bit more, which the slacker STA gives you.
Old-school "roadsters" often had a STA of around 70 degrees which is what I used on my extreme rad-yet-trad opafiets-style build.