Road a Surly touring bike - very sweet. My problem - I need a 48 & Surly makes a 50 and a 46. My road bike is a 50 but could be a tad smaller & be OK. I road a 52 & the standover height was too tall - I could stand but not comfortably (if you know what I mean...)

Should I go with the 50? Or size down to the 46? I've read that smaller is better with touring bikes but I'm not quite sure why.