I tried the Techno on in both M and L in the UK this summer. In all honesty there was little difference. The medium was a little tighter but still thought it would ride up just as much as the L on a crash so thought I would go for L as it is a little easier to get on and off over a wetsuit. I am about a 43" chest at the moment.
I've found if you go by chest size, they are too loose around the waist. And since they are vests (no sleeves) you can usually fit in one that has a chest size that is smaller.