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Don't think you'll get a reliable answer on the internet... depends on whether you feel the cold and what kind of pace you ride. For all the people saying they only get it out when it's under 5°c and wear it with a thin baselayer, there are others who feel the chill and like to layer.
(right now I'm sitting indoors wearing two merino t-shirts, a wool jersey, a scarf and a rapha softshell. I expect I'd overheat fairly quick on the bike but I'm still cold now... and overheating for me will mean taking off my scarf and dropping the hood of the jersey) -
Buy both sizes, use the one that fits your gear closet best, you have 30 day "Classic Guarantee" where you can use it, and if it doesn't work for you refund it. For the unused jacket you have 90 day refund if you don't use it. Simple =)
Hard to put a temperature on garments, that's why Rapha's descriptions now don't mention temperatures, and in the Q&A portion, the CS reps are very general with their responses b/c everyone is different.
Would a Rapha Ladies softshell be warm enough if it gets a tad colder than it is now? I tried on the medium with a fitted long sleeve base layer and it fit but I couldn't layer anything else. Next size up would be too big. V tempted in the current sale. My current Gore softshell is pretty snug but gets a bit chilly around 5 degrees.