There’s endless research and debate on best sleeping bag materials and the effectiveness of each. REI does a great job of quickly breaking down the options to make buying a bag less intimidating.
For our use, all-season rectangular sleeping bags have served us well over the last 5 years. I feel claustrophobic just thinking of the mummy shaped bags and absolutely require the added legroom of a rectangular sleeping bag.
Also consider the toe zipper functionality to open the bottom of your bag and help regulate internal temperatures. The combination synthetic/down fill bag from Eastern Mountain Sports have held up extremely well over time, having been packed into a compression bag well over 150 times. Only this year (after 5 years) has the fill began to leak out – just 1 or 2 “feathers” at a time.