Rapha, nice but a bit pricey?

I'm seriously considering splashing the cash on some assos bib tights. I'm Just over 6ft and around 31" waist, with a mega skinny ass. Would medium suffice? I've gone small on non-bib shorts before to get the fit, but I guess I need length of strap / leg too now.

Also... Rapha deep winter jersey - worth the £160? They look beautiful

I am 6ft, 65kg 29" waist and have a pair of large Assos bib shorts which fit and a medium pair which also fit (both S5 Uno)
The medium is a bit more snug, but both the straps and fabric have a good amount of stretch (much more so than other bibs I have)

Tights may have a bit less stretch if the fabric is thicker though...