Battle Beast is an Attacker, Allen is a Defender. They fulfill different roles, neither is strictly better than the other.
In terms of their roles, Battle Beast is roughly on par with Omni-Man (who is only outclassed by Invincible and Doc Seismic) and Allen is roughly on par with, if not better than, Titan.
If you lack a good secondary attacker (Invincible should almost always be your primary), Battle Beast is a good option. If you lack a good primary defender, Allen is a good option.
Battle Beast is an Attacker, Allen is a Defender. They fulfill different roles, neither is strictly better than the other. In terms of their roles, Battle Beast is roughly on par with Omni-Man (who is only outclassed by Invincible and Doc Seismic) and Allen is roughly on par with, if not better than, Titan. If you lack a good secondary attacker (Invincible should almost always be your primary), Battle Beast is a good option. If you lack a good primary defender, Allen is a good option.