Well besides for the alternatives expressed here it really depends on what conditions and where you are riding.
I prefer Primaloft over Thinsulate for a warming layer. It's a bit lighter, compresses more like down and is as warm as down with less puff. I have a great Primaloft jacket I bought a couple of years ago that quickly replaced my down jacket for cold weather conditons. Primaloft also does not stop keeping you warm when it gets wet. Big bonus in my book. It might break down though, it seems to me that this season it's not quite as warm as I remember it being when I first bought it.
Down, well you can't really go wrong with it. Super light, compresses down to almost nothing. If you are going on an expedition, you probably want a lot of Down items. The higher the fill number the better. I wouldn't buy anything down that is under 700 fill. Just remember if the membrane of the jacket isn't very waterproof or just gets soaked through in general, down is worthless for keeping you warm. Once it's wet your hosed.
Last edited by killclimbz; 12-27-2008 at 10:07 AM.