
Sleeping bags are like footwear - entirely a personal thing. If you're a warm sleeper, then you can get away with a lighter bag. If you're a cold sleeper, you'll need a heavier and bulkier bag.
I use a Snugpak Travelite, which is rated 'one season', weight very little and packs very small, and I can use it for everything except winter camping, for which I'll upgrade to one of several three season bags I possess. But then I'm a warm sleeper, so I can get away with it. A cold sleeper would probably suffer greatly in a Travelite even in a heatwave! So... for you... it's whatever works.