My experience with both warmers and bib tights is that sizes are all over the map. I have several pair at this point and the sizes range from L to XL to XXL between brands. I find that I must try them on before buying.

I only wear the warmers when I might want to take them off during the ride. So if the temperature is likely to get about about 50F, I'll wear the warmers. Otherwise the bib tights are the best bet.