Winter Bibs for Mountain Biking

Since you already have shorts, have you considered leg warmers? Pearl Izumi (for one) has a full-length fleece-lined leg warmer that I really like. If you're dead-set on full fleece pants, then I'd just make sure that you get a reputable brand. Crappy chamois is a ride-killer. Personally, I don't like the full length tights, I get too hot in them. Just make sure if you go the leg warmer route that you don't get them too small. You'll spend the entire ride pulling them up.

Disclaimer: This is opinion of a Georgia rider. Typically, the high 30's to low 40's is about as cold as I'll ride.

These Twin Six Thermal bibs are what I'm using this winter. Quality material with a good pad.

I use leg or knee warmers depending on how cold. Pearl Izumi are my preference. If it’s very cold I use both the leg and knee warmers plus two pair of bib cycling shorts to keep everything warm.

I have two sets of the Specialized thermal bibs. A 3/4 one, and one that is the full leg. I'm not sure on price because I get a discount through my LBS, but they are awesome. Keep me warm, and very good quality.

I wear thermal tights underneath regular padded cycling pants. It's good down to 40* or so.