We have both synthetic and feather bags. Both keep you warm. The feather packs a bit smaller but in a swag you wouldn't notice the difference. Have been told the feather bags should not be stored packed. Synthetic are easier to launder.
Have both Mountain Designs and Katmandu. Whatever you do, don't pay full price. Wait for one of their regular sales. You can get different sizes and shapes. Some are like a mummy shape, others are square, some are wider at the shoulders, etc, etc.
KB