Not all that many years ago *I* would have said, "Go with a down bag and get yourself a `quality' ground cloth & cover for it." But, have just recently doing a bit of research on some of the newer synthetic insulation that's `out and about' these days, I'd have to `vote' for some of the higher `high tech' stuff.
The *best* I found was a construction that used a *closed* stranded cell `bulk' / `dead airspace' insulation in combination with with a combination `reflective' & `windbreak' (Along with other factors, in the *highest* `high tech' offering, like `breathability' and other `strange?' `features'.) layer over the outer suface. Also, amazingly enough *not* in the priciest versions I ran across, there were even *some* that had gone to sort of a `flannel' or `polar fleece' liner for the inside. And `pound / pound', or `ounce / ounce' as it may be, even in the `higher priced spread' what one would pay for now for a down sleepingbag and what one would be getting even with the addition of the `ground cloth' / `cover' VS one of the new `high tech' sleepingbags is such that your `value' is now with the `high tech' ones. Both in what you `get for your money' (IE: What you can hold in your `grubby meathook' and see.) and `performance' (IE: When you crawl in for a good nights sleep.) *AND* just what it is you have to *lug along* (IE: Weight and size and just what kind of `performance' it has *for* that weight and size.)
*I* could carry *two* bags in the space, and actually *possibly* shaving some `total' weight off, too, that I used to carry *one* down bag! I *like* it! I'm starting to get a bit on the `frail' side these days and *anything*, short of giving up `firepower', I can do to reduce what I have to carry I'll go for! So.... Take a look at the synthetics *and* also take a good look at the `features'. I think that if you look at least a half dozen places that cater to `backpackers' *and* go looking for other places *other than* those `so-called' `backpackers' places and you'll be pleasantly surprised at how much `bang for your buck' you can really get these days.