Sleeping bags are sleeping bags. Quilts are quilts. I used a quilt but after several nights, I could not get used to the middle of the night draught from rolling, soo, went back to a bag.
The Summerlite is a nice bag and the temp rating is fairly accurate. The Ultralight is a good bag and is accurate to. But they are much different and designed for different conditions. My choice would be the 1#13 Ultralight for general good looks and function. Probably too much for mid summer. Probably not enough for late fall/early spring (OK with additional down and good padding in a tent or enclosed tarp, though.) Note that most good bags are rated for wearing a base layer of some sort in them, often a functioning set of long johns and socks. The down jacket will add maybe 10F, depending on your pad. Often a good pad selection will warm a bag by as much as 10F. Or cool you off in summer by about the same by simply removing it and using a 1/8-1/4" piece of foam. So, there are other factors to consider. Shelter will have some effect on temp, also. Fully enclosed tent? Open Tarp? I usually figure about a 5* difference. Winds...how much? In the woods? On grass or other high humidity ground cover making your bag damp? Anyway, I figure about 15-20F total. Then you really need another bag.
Not quite straight forward, get one over the other. But, it should give you a bit more to work with. The Summerlite may work well and save you some weight depending on where and how you intend to sleep.