Given the choice, I'd take the Summerlite, provided its a comfortable fit. It's 6 ounces lighter, better made (and made in the US), and I've read mixed reviews on whether or not the Golite Ultra lives up to the 20 degree temp rating. Which, by the way, you're not likely to need on the JMT in late July or August. If you want versatility, just unzip the bag all the way. Quilt/unzipped sleeping bag = same/same.
Plus, if you've never slept in a quilt before in temps under 40 degrees you may be in for an eye opener. Without a bivy sack, relatively wind resistant insulating clothing, or a solid wall tent drafts and wind gusts are very difficult to mitigate. Too high of a fiddle factor for the (maybe) 5% weight savings. And, I don't want to be conscious when I roll my lazy a$$ over.