Polyester is just fine, providing you wear some fleece underneath or wooly sweater, and that can get a little bulky. Down is great since you can have just a t-shirt underneath and will not be cold. Polyester with good lining (mocrofiber or fleece, etc.) can do the job, but I'd go for down, if you are petite and get cold easily. Just beware of cheap down, it does not take wear and tear well, the cheap down comes thru the lining after a few weeks of wearing it. You don't want to look like a chicken, dropping feathers here and there