The 686 are a tad more waterproof and probably a bit more durable and warm (never ridden the dc though). My cousin picked up some dc sneakers and they didn't even last a month, although they were from ross. Since I'm normally a 32-34 waist and the mediums are a bit big on me now I think at a true 31 waist the smalls should fit pretty great. Just my .02 though might want to wait for someone with some dc's to respond.
I already had some 09' smarty's which is why I picked up another pair off geartrade this summer. My other are still holding up excellent, I just wanted a spare pair incase they ever stop selling them. Unfortunately I never asked about the liner and just assumed it was included from the pictures, but it wasn't. Geartrade (aka backcountry.com) are pretty cool though. The item did arrive with tags still on, and after asking them about the liner they gave me $15 back (only $52 to begin with). I just ended up doing some research and getting: Under Armour Mens ColdGear Fleece Athletic Pants
There's a little box at the bottom of the page on geartrade that lets you ask questions. They usually get back to you really quick.
Also there's retail price and then there's the general price that everyone is selling them for. The 686's retail for $199 and alot of places are still selling them at that. The dc's retail for $229 but a lot of places are selling them for only $129. That should say a little something about how quality or popular they are.