I have one in Monogram Canvas and it's one of the smart casual bags I use when I go out. Mine is still fairly new so there's no slouching yet, but I imagine if it were to slouch it wouldn't be as obvious as on the Speedy. Epi would definitely be more stiff; then again the Sac Plat is a fairly structured bag to begin with, in whatever material.
I have a cerises sac plat and it is very structured, no slouching around here. I use it as one of my everyday bags and it can be cumbersome at times but I carry it like a speedy- hand held or in the crook of the arm and it's great. It fits all my stuff and I tend to carry quite a bit.