The question you may want to ask yourself is... "What kind of game do I want to play?"
Both games are fast and deadly when set up properly. Both games have that Steve Ritchie flow and feel. Both games are some of Constantino Mitchell's best work. Both games are classic early SS tables.
The difference is in the ruleset and gameplay.
Firepower:
Most people would say Firepower is about multiball. It's not. Firepower has no reason to start multiball other than it's a great gimmick. No PF multiplier, not jackpot to score... just great pin-theater and three balls whizzing around the PF. Firepower is about lighting the spinner and ripping it as hard as you can, as many times as you can. That's where your biggest scoring opportunity is. Simple and awesome!
Black Knight:
This game really is about multiball. 2x and 3x multipliers during multiball on 2 levels of pin-playfields will keep you entertained, focused and distracted at the same time. Drop targets and a lower bonus X loop will have you on your toes at all times.The left ramp to the upper left flipper to the ball lock is a double leveled full playfield figure 8 shot that is most righteous when sunk. Do you like your game more in depth? BK is definitely next level as far as rule complexity and layout is concerned.
The bottom line is... This is your game. To best find out which one "speaks" to you the loudest, I'd recommend that you play them both. Go with your gut feeling and get the one pinball table that you want to play over and over and over and over and over...