Loki, I think you will find that lighter sweaters will work down to about 32F. If you plan on colder temps, around 20F, you need the heavier shell. They are different and have different purposes and temp ranges.
Down Sweaters are really excellent in sleeping bags. Low bulk, very warm and good for about a seven to ten degree boost to a summer bag (40F.) Because they are down, you can sort of adjust the upper temp range a little with unzipping all the pockets and down the front. They are great for midnight runs to the local stump. Waking up in one and getting out of the bag is nice because you don't get a temp shock. For hiking in, even at 20F, they are just too warm and tend not to breath well enough to stay dry. In rain or heavy mist, you really cannot use them. If they get wet, the down will collapse, leaving you cold...especially at night if you rely on them. If you need one under a shell, you are looking at very cold temps, indeed. While they usually have some sort of DWR, they are not shells, nor do they work all that well as one. I have never hiked the PNW but I believe it is wet...even in winter like conditions with wet snow, sleet and stuff falling off trees. Sweaters were not designed to cope with this and a shell will be better.
If you get a shell, you cannot plan on it to sleep in. If it rains(or wet snows,) you should not put it in your bag. But, if you need a shell, chances are it will be below 30F for the entire time you plan to be out. Water is not that big of a problem when it is ice or snow...you can usually brush most of it off and it does not saturate your bag over two or three nights. Older military arctic stuff had actual plastic pouches around the down to prevent moisture from penetrating the down. A good system, but really heavy. If all you are looking at is hovering around the freezing point, I think the down sweater would actually be more usefull. But you should couple that with a fleece (or shell you can unzip the lining from for hiking in.) Of course, winds will play a lot into your needs. Cold weather hiking is a matter of constant heat regulation, opening, closing, taking it off, putting it on as you hike. Water, sweat, is THE problem. Generally, you are better off hiking cold than allowing your insulating layers to get wet with sweat.
Shells may or may not have baffles. Sweaters never do. Same for hoods, though some sweaters have them. A good shell will have a lot of features, primarily pit zips, hood, high collar, dual fasteners, outer pockets (outside the insulation,) inner pockets, pass through pockets, thigh skirts/and adjustments, and more. A sweater has none of these niceties. Well, maybe a bottom adjuster. These are designed to keep you tighly sealed aganst the enviornment. The sweater relys on the shell for that protection. They simply are designed to supply insulation, maybe a few features to make them a stand-alone jacket. Again, thay are designed for two different purposes. Generally, a shell is purchased one size larger than what you would normally wear. This is to allow extra insulation under them. Sweaters are designed to fit you rather tightly, maybe with your base layer and shirt under them. I don't think the complaint of a tight neck is a problem, you likely are looking at the wrong size. The fit being poor on the rest of your body is likely due to lack of insulating layers under it. Maybe not...it is hard to say from a distance.