Submarines certainly have both offensive and defensive (as hit soakers) value, but strategic bombers offer greater flexibility due to their movement and attack values.
If, for example, your opponent buys a bunch of subs (for use as hit soakers) in anticipation of an attack by your fleet, you can hit them with aircraft only and take the subs out of the equation.
Even if your opponent doesn’t go that route, well-placed bombers can be used for strategic bombing raids which enable you to out-spend your opponent on naval purchases and break the “stalemate at sea” in a few turns. On the other hand, if your opponent fails to buy enough destroyers, subs are a more cost-effective way to achieve the same result.
In my opinion, it comes down to this: If your opponent has few destroyers, buy subs. If he has many destroyers and/or subs, buy bombers.