Unless the bag has a
certified rating or you can find a proper review, suspect BS. Ignore 'extreme' ratings marked on bags.
I'd suggest, if you want to get the best buy, to post a link or description to the prospect on here, someone will help. A '-5C' advertised bag could be generally cold at +5c
If you have the money a down bag will be lighter. If you have conscience for the geese, some 'ethical' down products are available.
Or(?) a quilt may be more useful in your situation?
viewtopic.php?f=27&t=21674&p=316272&hilit=UL+Quilt#p316272And considering utilizing clothing in the mix as mentioned (so you don't over-buy with the bag).
I use a -1c sleeping bag for 3 seasons.. even +4c if there's a good forecast.. (or until it was nicked recently),
but always with some puffy clothing on hand jic. The -1c bag cost $280(US) and weighs 700g's