Over the last year I've moved from a cardigan (used to travel with the Lands End Supima one listed above in a post which has now disappeared??) to a Dri-fit 1/4 zip pull over. I find them as warm as a cardie and much, much less bulk. I took 2 ( black and gray which went with my capsule wardrobe colors) on my last trip to Paris, Belgium and Netherlands in March and April and along with a puffy vest (easily compressed) took me down to temps that were around 32F. I also took the cardie with me but never really wore it as the dri-fit worked for day and night. I took one with me last year to England and Scotland (May and June) and started out with it on every day.
I've been using the Ideology brand from Macy's because I've gotten both on sale for around $20. There are more expensive ones from some of the athletic apparel companies as well.
The puffy vest I got at Costco last year for $20 or so. I don't think they have them any more - haven't seen them at least. Many companies make a puffy vest and they compress pretty flat. Also good for use as a pillow on the plane, lol!
I always travel with a waterproof layer as well. I suspect you will have that along too, but that combined with the dri-fit was a warm combo for me.