4everplan had it right. Time of year and whether you sleep cold or warm are the big considerations.

For reference, I have a WM Summerlite (rated to 32 degrees), and I get cold easily. I've only used this bag on two trips, both in August, in Oregon's Strawberry Mountain Wilderness and Diamond Peak Wilderness, and a couple of sleepovers at a friend's. One night it got down to 28 degrees and I needed long pants, shirt, and fleece jacket in the bag. All the other nights it probably didn't get down lower than 40 degrees and all was OK. So, if you tend to sleep cold and plan to visit other areas too, the Summerlite might work for you.

No doubt you've browsed WM's web site. They have several sewn-through bags for warmer weather, and which can be used in combination with another bag for colder temperatures. I notice there are several warm-weather bags listed on their price list that aren't shown anywhere else. New items, I suppose...might be worth a call to them to investigate.

I recall reading in Colin Fletcher's "The Thousand Mile Summer" that when he was hiking through Death Valley, nights were warm and he sent his sleeping bag ahead, thinking he wouldn't need it. Then one night it dropped to 55 degrees, which seemed quite cold after the previous nights, and he was unable to sleep, so just got up and moved on in the dark. I think that was April or May.