Can’t tell from your username whether you’re male or female but with regard to comfort levels be aware that on average females sleep colder than males so will require a warmer sleeping bag than males to stay comfortable at the same external temp. My wife’s summer sleeping bag is rated down to -10. If the temp is anywhere near 0 she steals my -20 bag.
Down sleeping bags are more comfortable over a wider rang of temps compared to synthetics. Down sleeping bags generally last longer and are less bulky than synthetics for the same amount of warmth.
Features to look for are neck/shoulder baffles with a drawcord hood as these give you quite a bit of temp control. Zip baffles to prevent cold air coming through the zip and an anti snag strip behind the zip to prevent it snagging the baffles or inside of the sleeping bag. If buying down look for down that’s been treated to make it resistant to moisture also check the material for water resistance. This helps protect the down from moisture which reduces it’s insulating properties. This is less important for synthetic sleeping bags as they’re less affected by water.
Sleeping bags get more expensive as they get warmer, lighter, more compressible, more durable/hard wearing and the number of features they have. You’ll need to choose which of these are important to you and how much you want to pay to select the sleeping bag which is right for you.
Makes with a good reputation are Rab, mountain equipment, north face, mountain hardware, snugpak, mammut.