Since you want this quilt for a possible thru-hike, I would look for a rating around 20F (not that I have experience with long thru hikes). That will keep you realistically warm to about freezing and then on colder nights you can wear your insulating jacket to bed.
My suggestion is to look at a quilt over a bag. I made the switch last summer and I was nervous about whether it would be a pain to keep the quilt tucked in around me. As it turned out, it was a very painless switch and I actually prefer using a quilt. It's easier to get in and out, which makes those middle of the night washroom breaks that much less painful. It's also easier to reach an arm otu and check the time. Keeping it tucked in isn't a pain and I switch sleeping positions a lot. A quilt is a lot more like sleeping in your bed at home than a sleeping bag.
It's not the lightest possible quilt, but the GoLite 3+ season quilt (rated to 20F) is well made and very affordable:
The retail price is $275 and GoLite occasionally has online sales around 30-40% off. I wouldn't be surprised to see one before christmas. The last one was for the national trails day this summer. This quilt has 11.5oz of down, which should keep you quite warm. GoLite also has generously wide quilts so they are easier to keep tucked in compared to more radical quilts with a slimmer cut.
The Katabatic Sawatch (15F) is another nice 3 season quilt but with 15oz of down you might find it too hot in the summer...a better option might be the Katabatic Palisade (30F, 9.2oz down) with some overstuff (2oz is $18) if desired. With 9.2oz of down, you'd want to have some decent other layers on hand for the shoulder seasons.
This quilt is very light (17oz) and it's $315. I'd get it overfilled it with 2oz, you'd be looking at a 19oz quilt for $332 which contains 11.3 oz of down. That's a pretty sweet package. The only downsides here are a rumoured slimmer cut than the GoLite quilts, and the price. The GoLite 3+ season is 5oz heavier for the same warmth, but if you could get it for 30-40% off ($165-$192) then you'd be looking at a big savings compared to $332.
If you're really looking for value, the GoLite synthetic 1+ Season quilt (40F rating) is normally $120, but BentGate has it on for $101 and then you can use this coupon code (rockandice2010) on top of that for another 15% off. That would give you a 40F rated, 24oz synthetic quilt for $85 and free shipping too I believe.
That would be a sweet quilt for the summer, but you'd want to layer up well for the shoulder seasons. When it's going to be cold I use down pants (7oz) and a down vest (7oz) in addition to the montbell UL down inner jkt (8.5oz) in my 30F bag. You'd want something similar. With the right layers, you could put together a pretty versatile system, but carrying a bunch of pieces is always heavier than just carrying a warmer bag unless you want the insulating pieces for use outside of sleeping.