Buy the puffer first. It's already snowed twice here, and it's not even Halloween yet, so chances are very high we're in for a rough winter.
My Jcrew coat has thinsulate and like LE it isn't as good under 40 degrees - especially if it is windy. Also, it spots in the rain or snow. With that said, I think some of Jcrew coats have much thicker wool. If you opted for one of those, and added thinsulate, you might be okay - but I ordered a coat like that and returned it - not just because of the fit, but because the coat was too darn heavy!
On the other hand, my puffer can be TOO warm. I can wear it in the coldest weather of all, with nothing but a slinky dress underneath, and be toasty as can be. If I'm not going to be walking outside much, I will skip it and wear the Jcrew and be chilly for a bit, rather than overheated indoors. But I think you spend more time commuting outdoors, so the puffer - even a too-warm one - is probably a wise investment.
