I also prefer a 20* bag for 3-season hiking. At high elevations in the Rockies and even in the high Cascades it can freeze or snow even in mid-summer, and in the fall it can get down in the teens. Lots of folks, though, get 30-degree bags and layer on more clothing inside.

Unfortunately, there are three items everyone wants in a sleeping bag that just won't go together: warm, light, cheap. You can pick only two! Start saving your money for a really good down bag: Western Mountaineering, Marmot, Montbell, Feathered Friends, Nunatak, Valandre. It's a big investment but, properly cared for, it will last a lifetime.

If you can't afford a higher-priced down bag, for now look at the Campmor Down Sleeping Bag. It's more a 25* bag than a 20* bag, but it's definitely lighter than the 20* bag you're looking at, which probably also has an overrated temperature rating (as do most inexpensive bags). Many on this forum and others recommend it to those whose budget does not lend itself to Western Mountaineering, et al. (Mine doesn't either, but I bit the bullet and it has been well worth it!)