The last greatest factor to consider in selecting a bag is the insulation that is used in that bag. While both will do the work to maintain you warm, they both have their pros as well as cons. Some kind of extra padding under the bag is very advised.
Rectangle-shaped sleeping bags go as much back as the late 1800's when a mail order catalog pioneer Pryce Pryce-Jones came up with the suggestion to attach 2 rectangular carpets together with ties. He is taken into consideration to be the innovator of today's contemporary rectangular sleeping bag design. After the war he had several of these bags still left as well as made a decision to market them to the public.
There are primarily two types of sleeping bags. One is the mommy bag and the various other is the "traditional" rectangle-shaped resting bag. Currently mind you, ain't nothing incorrect with old school in any way. It might be your preference once you uncover just what it's like verses the mommy bag.