montbell bags are great, I definately recomend them. to determine which bag you will want to take, It really depends on the other gear you are taking..
are you bringing a tent or a tarp? what other clothing will you be bringing? what kind of sleeping pad will you be bringing? do you want to use the same bag for your entire thru hike? or do you plan on mailing yourself a warmer bag for the sierras and for the finish?
Im always excited to hear when people are planning for a thru-hike, I would recomend posting a gearlist, so people can make recomendations.
this always helps me when planing for a trip.