I've never tried the Bellas (like you I'm tempted)- but I have a number of other Terry products (T-short, knickers, tights). The mediums fit me really well. (I'm a medium in most of the other shorts I have including Shebeest- I've never tried Sheila Moon). I'm 5'6" and 135-140 pounds.

I wouldn't get too caught up in the measurements (they seem to vary manufacturer to manufacturer). Terry allows returns- order both sizes and send the wrong one back (return shipping on a pair of shorts will be pretty cheap and you can try on both).