Battlecruiser or fast battleship depends on the mission

Which is better, a battlecruiser or a fast battleship?

It depends on the task: fast battleships for battleship combat, battlecruisers for commerce warfare and some fast escort roles.

The answer refuses a universal winner because design follows operational purpose. Heavy line combat against enemy battleships favors the fast battleship's protection and staying power. Commerce raiding and hunting surface raiders may reward the battlecruiser's different balance of speed, firepower, and armor. Carrier escort also depends on how fast the group must move and what opposition it expects. The useful comparison is therefore mission against mission, not ship label against ship label.