In this situation, you can't pick both. The offensive measures we are discussing exist by breaking the defensive measures. The problem is breaking the defensive measures breaks not only their defensive measures, but ours as well. Hence Snowden's analogy to making a secret entrance to all bank vaults, even ours.

In this case, the idea is that the NSA will keep knowledge the secret door to the bank vault to themselves, and we'll all be secure. The problem with this is that other countries can inspect the bank vault and discover the secret door for themselves, or steal said knowledge from the NSA via espionage. Now our bank vault is just as exposed as theirs.

If offense includes things like trying to ban or secretly weaken defensive measures like encryption, then that doesn't work out so nicely.

I think offensive vs defensive is a very easy choice. We want to prevent our systems from being compromised, but don't have any business provoking attacks by breaking into systems. Invest in defense.