You might have figured this out by now, but the reason that you'd have to worry about loft loss is because of the synthetic insulation. Synthetics don't last long. All of the other options we're talking about are down and you won't have to worry about losing loft (as long as you wash your body oils off the down periodically). As far as wetness, just use common sense to avoid getting your bag wet.
When talking about quilts v. bags in these forums it's easy to start a bit of a flame war... I offer my opinion on the matter in peace. People talk about the lighter weight of quilts, but as you mention several bags are very similar in weight--maybe even lighter! I prefer the versatility of a sleeping bag. I can totally zip myself in for warmth if I want, or I can unzip it all the way and drape it over myself exactly as a quilt user would do. There is zero functional difference when used that way. Ultimately, some people prefer the openness of quilts while others prefer bags. I would recommend looking at the WM Summerlite, 32*F rating, 19 ounces for a 6 foot length with a full-length #5 YKK zipper and a hood. If it's hot, unzip fully and use as a quilt. Or just use as a bag but shift most of the down underneath you before bed. The WM LineLite might be an option, too, at 45*F and 14 ounces. Have fun making the decision!