But, as you note, the Pinnacle is available in a regular nylon shell ($309 in regular size) or with Dryloft ($399). Dryloft, of course, is a light version of Gore-Tex that's designed for use with insulating products such as sleeping bags and down parkas. It's designed to offer more water-resistance than other shells, without sacrificing much breathability.
Which is best? I don't think there's a clear-cut answer, but I generally believe that the Dryloft is an unnecessary expense. The non-Dryloft material that Marmot uses, for instance, is a very light fabric that's treated with NikWax for excellent water-repellency. And it breathes extremely well. Under the conditions you describe, nighttime dew might dampen the shell a little. But you'll get no more moisture inside the bag than relative humidity allows, and 20 minutes in the morning sun will dry it right out. That isn't to say the Dryloft would lead to mildew problems as you fear-it won't. Ultimately, while a Dryloft shell really is nice for users who might have to contend with snow blowing inside a tent, for all-around bag performance, I think a nylon or polyester shell makes the most sense.