Daniel, thank you for the question.
I think it will depend on your location/venue you will likely find yourself in to survive [which tools/equipment you will likely use/need?] and then get a survival bag to suit your needs. [pockets/water tight/size etc,]
Maybe first start your research at the other end and see what you want to take with you in the bag [weight?], and then get a suitable bag for all that?
To be able to use your survival tools well [skill] , and to 'read' the environment is maybe more important than the type of bag you have in my opinion.?
Amazon offer some very good bags in a verity of shapes and sizes and it's maybe worth to have a look there.
If you can maybe give a bit more info on your likely survival situation [rough camping?] , there are a lot of highly experienced members on this forum that would also be willing to help and assist out of their vast experience.
All the best, and remember , it all first start with an inquisitive and positive mindset!