Crystal B is better in terms of disabling ships followed by killing enemy crew thanks to 3 crystal lockdowns, but Mantis B is better for killing crew/rooms sooner.
Crystal B pros:
3 crew members, and all 3 are crystal. This means you can disable a room without interruption from constant sabotaging by using their abilities to lock the room.
Starts with 7 out of 8 systems. If you use cloaking, you can use it to dodge shots 60% of the time if you send all 3 to board.
4-man TP
Free 40 scrap from selling the "crystal vengeance" augmentation.
Crystal B cons:
No weapons, though this is really a concern only if you're fighting automated ships (or lanius, but you should have a weapon to take those out by then).
7 out of 8 systems already picked for you means you get to pick only 1 system of your choice.
Weird room layout, especially near the cockpit and along the weapon area. If one of those rooms along the way is locked down, you'll have trouble getting through the locked room just to reach the farther rooms.
