Double-Check My UQ Logic

[Hiker9]

I will have the funds shortly for investing in an under quilt, trying to decide what I need. I'm leaning toward a 2/3- or 3/4-length summer down UQ. Assuming a summer bag is ~35-40° and a 3-season is ~15-20°, here's my logic:

1. My backpacking range is the southern Appalachians: north Georgia, the Carolinas and Tennessee. I go from mid-spring to late fall when temps are moderate. A summer bag would be warm in late March at Black Balsam Knob but not roast me during July on the Chattooga river.
2. The weight difference between a summer and 3S bag is negligible, about 4-5 oz. The family budget is tight though, and summer UQs run about 20-25% less than 3S.
3. I'm an average sleeper temp-wise: not hot, maybe a slight hair towards cool. I'm an experienced backpacker so I know how to stay warm at night. I plan my sleeping clothes and/or gear so I won't have to use my jacket. I also know to pack a thin pad and to use that jacket for my knees and feet if needed.
4. I'm rarely out in what we southerners think is deep cold (20-30°), maybe once every two years. However, if I wanted to see some snow, I can pack a thicker pad or some extra bottom insulation like the modern GI poncho liner or a 15-20° winter sleeping bag to use Pea Pod-style (with the UQ inside, à la Shug).

So where are the holes in my logic?