My family and I are going to be backpacking in Dark Canyon this May. From those who have been in the area this time of year, what is the weather like at the bottom of the canyon? On the rim? Suggestions for sleeping gear and shelter?

Any insight would be appreciated.

The weather can be highly variable that time of year. Could be 100 down in the canyon, could snow in the upper reaches on the rim. The one guarantee is lots of wind. Depending on where you plan on camping, packing for nights close to freezing, hot days, and being able to stake your shelter well in sand is a good approach.

I have a 15 degree bag and a ~40 degree bag. I just got both so I'm not sure the how cold I can go down to in them (I will be trying the 40 degree bag out soon). Do you think I could get away with the 40 degree or should I stick with the 15? I obviously plan on checking the weather forecast before we head out, but I've never hiked in a climate like that of dark canyon before.

Depending on the elevations at which you plan to camp, the 40 should be fine.