Let me say up front that I don't know much about WM bags, but I want to point out that the temp rating to look for depends very much on not just the location but also the time of year and the specific weather forecast for the time you'll be there. Just looking at average lows in Death Valley and Joshua tree, they range anywhere from 39F and 35F, respectively, to 88F and 70F, depending on month. And, that's just the averages. There's always the potential for colder weather. Check the forecast before you go.

EN rated sleeping bags have two ratings, the "comfort" rating and the "survival" rating. Guess which one is more prominently advertised on the packaging and online. If you said the survival rating, you'd be right. So, a "20 degree bag" means it will keep you alive down to 20, not comfortable down to 20.

Another thing to consider is how warm you specifically sleep. For example, I sleep very cold. Event though I use a bag with a comfort rating of 32F, I'd never want to use it down to that temp without wearing extra warm clothes inside of it (puffy jacket, long johns, wool socks, a cap, maybe sweat pants). Other folks are the exact opposite and can use a 40F bag down to freezing. If you don't already know, figure out how warm you sleep and take that into consideration while you shop.

Sorry there's no simple easy answer here.