Loc: Portland, OR
For a bag, the main criteria are the bag being warm enough for the coldest night you are likely to encounter, the overall warmth-to-weight ratio, and selecting an adequate girth, especially if you are broad-shouldered or shift around a lot in your bag. Zipper-length is sometimes an issue, too. A too-short zipper may save a few grams, but may make the bag less versatile overall when you can't ventilate as well on a warm night.
You probably can't go wrong with WM bags, in terms of quality of construction -- after you've chosen the proper bag and features for your personal needs. I am also intrigued by the Montbell UL Super Spiral, which has diagonal baffles and stretchable seams, but I have no experience with one. I don't think they make one with a 20-degree rating, but offer it in 15 and 30 degree models. I can say that Montbell's construction quality is quite good for their clothing, which I do own and use.
The main criteria for pads are comfort, insulative properties, and weight -- in that order. If you can't get a good night's sleep on it, it doesn't matter how light it is -- and that includes not sleeping well because your warmth is being sucked away from below. Bulk is also an issue for some pads more than others, but that is highly individual to how you pack. There's a custom-pad maker who has garnered a lot of praise from other forum members (Kooka?). They can chime in on its merits.
Because the comfort part is #1 on the list, it really doesn't matter much what I use, because what I use may not work for you. You may sleep like a log with no pad at all or have a princess-like need for a 3" mattress.
Best? That's tricky. Where and when do you hike and what has worked best for you in the past?
"What has worked for you in the past?" Great question.
I just have an Alpine 20 bag now. I don't use a pad. It is comfortable on the ground for me to about the mid 20's. In the hammock, to the high 30's. I'd keep using it, but I want to cut weight and size.
Loc: Portland, OR
I take it the Alpine 20 is a synthetic fill bag. If you move to a down-filled bag (especially with the higher fill-numbers like 800) you'll discover that the down compresses underneath you to the point where there's little there but a couple of thicknesses of nylon. In other words, nothing. At low temps this will never do. Too much conductive heat loss that way.
But, because sleeping with nothing but your synth-fill between you and the ground worked fine for you, it does sound to me like you could easily get away with a closed-cell foam pad, or else a relatively thin Thermarest inflatable.
The advantage of a closed cell is they are very light weight and don't spring leaks. Many UL backpackers incorporate them into a system where the pad serves as the 'frame' of a frameless pack. Also, they are very cheap compared to good quality inflating pads. If you sprang for one (after trying it in the store, of course) and changed your mind, you'd only be out $25 or thereabouts.
The Z-Rest is pretty popular. It has an "egg-crate" design that sort of nests with itself when folded up, allowing it to pack smaller while maintaining a 'thicker' depth profile. The main closed cell alternatives to it are blue foam (cheap and generic) and Ridgerest.
I agree with Aimless about pads. You've already picked a great bag in the Ultralite. (However, if chest girth is an issue - it was for me - look at the Alpinlite: 5 additional inches in circumference for 2 or 3 additional ounces; same temperature rating.)
If you are looking for a little more comfort, I like my 48" NeoAir; I put my empty pack under my lower legs (which simultaneously solves the problem of where to store it in a narrow solo tent.) I'm going to get the All-Season Neoair and try it this winter - I'll know how it works by the time you're ready to get a pad.
IMO, you can't go wrong with Western Mountaineering. They are pricy but worth it.
As for pads, really depends what you are doing, and how well you sleep on them, and how cold it is.
Me? three season I am normally using a hammock with a blue foam pad in it, or if I'm ground dwelling up high, I use a torso length Thermarest prolite. The pad on the ground is nowhere near as comfy as my hammock - but it's lighter when I'm doing long days with no trees to hang on. I might eventually decide to try a shorty neo-air. There are lots of good options out there to try - but your sleeping bag will be your biggest investment.
Winter I use a big agnes wide insulated air core with blue foam on the top and the bottom of the pad. but my winters can be a bit serious.
If you do well on the ground alone, you probably don't need a lot of pad. Try a torso length z-rest, prolite, or neo-air. all of those options will be more comfortable (and warm) than bare ground and will not add a lot of weight. put your empty backpack under your feet and you're probably good.
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