Not all shingles are created the same. some are better and some are not. Your budget, and your future plans for the house help with your choice.
I can not vouch for their price estimates, but the flat roof material discussion is pretty good.
Not an endorsement simply a good example of what a major supplier feels is needed.
Still not an endorsement but this is what Mr. GAF thinks you should look for when evaluating your current roof for a replacement.