Shall is infers destiny; where Will infers directive.
"He shall become a very good football player" "He will become a very good football player" Shall implies that everything is lined up and it will most likely happen. Will implies that someone will see to it and it can't not happen.
The standard for a document that specifies the requirements of a system is that something 'shall' do something else (E.g. The motor's speed shall not exceed 95%). The requirements of a system are absolute and final, therefore I would say that 'shall' is the most strong of the two.