If you feel really invested in spending a lot of time backpacking in the future, you'll probably find a way to justify buying another bag eventually, so it might be a better idea to buy one with a lot of versatility now that will still be a part of your system later.
I imagine the most minimal and UL system that would handle true 4 season use would be a 10-20 degree sleeping bag like a WM Versalight, and a 30 degree quilt (check M. Verbers perfect gear page). Quilt in summer, decide according to the forecast and trip in spring and fall, bag in winter with layers or paired with quilt. I'm with Eric, go with a full zip UL 10-20 degree bag now, like a wm versalite or alpinelight and then see whats available for UL summer quilts in like a year or two. I'd bet there will be a number of great options in like the 13oz range by then, considering quilts are just starting to really catch on.
I was making the same decision a couple months ago with idea that over the next 2-3 years I'm going to try to thru hike at least 2 trails, do an extended summer trip in south america and lots of 3 season stuff here in the CO and MT rockies. I went with a katabatic sawatch. It provides about as much warmth as a 15 degree sleeping bag except its 9 oz lighter and a lot more compressible then even the lightest of the 15 degree sleeping bags. I've car camped with all my windows open in 5 degrees without extra layers and only the tips of my toes were a problem (this always seems to be a problem though!) I know thats not a good indicator for long term backpacking use, but I can tell you for sure that the 15 degree rating is accurate. So basically, if you're looking for an ideal 3 season setup I'd consider a 15-20 degree quilt. For 4 season, I'd start with a 10-20 degree bag.
Oh, the calculus of UL gear shopping!
edit: sorry, didn't see your last post, Paul. IMO, the exta 6oz is worth it, esp. if you actually intend to use it for light winter stuff. I think a 9-11 oz fill is good for three season with layers, but when you're really pushing the limits in shoulder season, its hard to know exactly how much extra to bring. For the most part I've used a WM highlight for 3 season use in the rockies for the past few years with layers. Its worked out fine, but to be on the safe side I find myself bringing a lot of extra stuff like down booties, and a warmer/heavier coat than I probably need to around camp. Even then, I'd occasionally wake up cold. Spending the extra 6oz of weight on sleeping bag down is more efficient warmth than in extra layers of cloths and it will also keep its loft better on longer trips with accumulated moisture. Also, in summer you're better able to get away with just bringing a super light vest for torso insulation which could easily make up for 6oz in the bag. Hope that helps and good luck getting your system together.