Goin to CO in November chasing muleys on foot. My brother (not going) and I both have Gore Tex jackets with liners. My liner is synthetic filled and his is down filled. Any reason to take one over the other?
Slip

Never been one for built in liners, much prefer layers (fleece and such). YMMV

Both liners zip out. I'm bringing his shell b/c it's more of a parka where mine is a wading jacket. I'll have fleece layers anyway so I may just bring the shell.

I'd go synthetic because if down ever gets wet, you're screwed.

If it's intended for really cold weather, like sub zero, I go with down. If use is for damp weather I perfer wool.

I just picked up a Patagonia down liner jacket. I used it on a sheep hunt a couple of weeks ago and it was warm as toast and very light. It also compresses into a very small stuff sac. If you wear it under a parka, should be fine in wet weather.

I'm packing my bag right now for my trip to Colorado this October. I'm of the school that layering will be better than most other options.

Depending upon the weather/temps I add or take off fleece jackets, wool sweaters, or wool blend shirts. The temps can swing widely in Colorado....I've started the morning at below freezing temps and then sweating and in shirt sleeves with 60 degree temps by afternoon....

Down is warmer than synthetic for the same amount of fill. Down will compress more and thus be less bulky. Will be lighter too.

BUT as tolt says if you ever get it wet from either rain or sweat you will wish you had used something else.

That is my experience anyway.

BCR

I switched to a Filson wool vest years ago, works well as an underlayment garment (in wet or dry weather).

If you want the lighter option, go with down.

If you feel you may have problems with moisture/rain, go with the synthetic.

Figuring you'll probably encounter some snow, which may lead me to believe synthetic would be the better choice. Also, take into account all the sweat you will be generating while hiking. Down can retain that moisture and in turn lose its insulating abilities.

Synthethic. You're gonna be climbing a mountain and sweating like a pig. Then when you're on top, you're gonna be sitting still for a couple of hours or more spotting. You'll be down to your long john top going up the mountain but you'd better have several light layers to put on as you cool down.