thanks for the tip, momentoftruth, but i'm still unsure. for reference, here i am in the regular fit, size medium. having just tried it on now, it's sloppier than i remembered - flaring quite noticeably in places and bunching a little in the upper chest. i'm pretty tall and equally thin, yet it's still too long. (and yes, this is post-hot wash / dry so it's done all the shrinking it's going to.)
as people have mentioned staying TTS with the slim fit, given the fit below in the medium, i'm pretty tempted to opt for small and just stick to cold wash / line dry as i do with most my clothes..
Warning: Spoiler! (Click to show)
go down one size at least. maybe even two?