The main difference, of course, is the width of the Megalite vs. the Summerlite. Many lean people don't like the extra width of the Megalite because it adds weight, and takes extra time to warm up the space. On the other hand, larger people like myself want the extra Megalite space.
The temperature difference< I believe, is due to the Megalite having a passive top collar, which the Summerlite does not. I think, a minor detail. The extra down in the Megalite is due simply to the extra (material) space that has to be filled. Each still specs at 4" of the same fill power down.
If I were a lean person, I would wear clothes in the Summerlite to add warmth during the short shoulder seasons. When those seasons pass, I think you would need a warmer bag than either the Summerlite or Megalite.
I had both the above bags (still have the Megalite), and like them both. Anything WM is great. But I now prefer quilts for their light weight, ample space unless cold, and thus their overall flexibility and comfort.