I, too, have both the Summerlite and the Helium. I mostly use the Helium because I am a cold sleeper. On my scales, the Summerlite weighs 19 oz and the Helium weighs 28 oz. For me, the nine ounce difference is less important than is sleeping warm.
I recently talked with the people at WM about having a few ounces of down added to my Summerlite. They recommended adding 2-3 oz to provide extra loft and to keep the down from shifting. The cost would be about $50. After the addition of more down, the Summerlite would weigh about 22 oz and would probably keep me warm to 32°F. Getting a 32°F bag and adding a bit of down might be worth considering; I'm planning to do it.