What temperature the manufacturer labels the bag for often has little to do with what temperature the bag will take you to. The only laboratory standards for sleeping bags are the European Union's EN 13537 ratings. These tests use a dummy wearing long underwear and a knit cap and "sleeping" on a 1" closed cell foam pad.

There are three sets of EN 13537 ratings. "Comfort" is for the "average" woman (whatever that is) and cold sleepers. The "Lower Limit" is the lowest temperature at which the supposedly "average" man (warm sleeper) can be comfortable. "Extreme" is the lowest temperature at which you hopefully will not be dead of hypothermia in the morning. Of course, manufacturers who don't sell their bags in Europe don't have to use any standards at all. Even these tests are very approximate and have been subject to criticism. However, they are definitely more accurate than the fictional ratings of manufacturers' marketing departments and do provide a fairly objective basis for comparison between different sleeping bags.

Be sure you know which EN ratings the manufacturer or retailer is using! Some (especially retailers who don't understand the ratings) use only the "extreme" rating in their advertising. Also note that the sleeping pad you use makes a great deal of difference, especially on cold ground! The 1" closed cell foam pad used for the tests has an approximate R value of 5, warmer than most common sleeping pads.

The best place to find the EN 13537 ratings is the manufacturer's website. Looking at the Marmot website, the Helium is EN 13537 rated at Comfort, 27.7*F and Lower Limit, 16.3*F.

For some reason, Western Mountaineering doesn't list EN 13537 ratings on their website, even though they do sell their bags in Europe. You'll have to look on a European website, such as this one.. It shows the EN 13537 rating for the WM Alpinlite and Ultralite as Comfort, 28.4*F and Lower Limit, 15.8*F (converted from the degrees C listed).

In other words, the Marmot Helium is quite comparable to Western Mountaineering's 20* bags. One difference is the absence of a draft collar in the Helium, which is why I spent the extra $$ for Western Mountaineering. To me, a cold sleeper, that draft collar adds at least 5*F extra sleeping comfort (very subjective!) and allows me to ventilate the hood so it doesn't get wet or frozen from my breath. The weight difference is minimal (3 oz.) when you compare the Helium with the comparably sized WM Alpenlite. The WM Ultralite (which I have) is lighter, but it's a narrower bag and may be too snug for bigger folks.

Be sure you get a bag with full-length zipper so you can ventilate the bag on warm nights. At higher altitudes it gets pretty cool most nights. In hot spells in summer in the Oregon and Washington Cascades, I've often started out with my sleeping bag unzipped as a quilt, or sometimes with me on top of the bag, but by morning I'm usually inside my WM Ultralite and most or all of the way zippered up.

If you can afford only one bag and need to take advantage of sale prices, the Marmot Helium (with full-length zipper) should be fine. Take good care of it and be sure you keep it dry!