At low levels no at mid to higher levels yes,absolutely the cantrip is better then an extra unless your dealing with distant minions.
Say your weilding a two handed weapon that deals d12 damage and have a strength mod of 5. Level 17. And your a paladin.
Two attacks for a Paladin means 1D12 + 5 + 1d8 radiant damage + 1D12 + 5 + 1d8 radiant damage.
One this Paladin using Booming Blade does 1D12 + 5 + 1D8 radiant damage + 3D8 thunder damage + another 4D8 Thunder damage if the Target movies. So either you keep the target from moving at the price of doing slightly less damage then 2 attacks, or it moves and you do more damage then two attacks.
The real benifit is with the warcaster feat, which would allow you to use Booming Blade as a Attack of Opportunity. This means the target abaddons moving, or gets nailed by the bonus thunder damage.