[Jon Stratis]

Based on my experience, Butterfly is much easier all around. Swerve is more complex to produce, more complex to program, and potentially more complex to wire (depending on exactly which swerve design you're talking about, and concerns about wires rotating with the module).

In terms of design and production, building a swerve module properly with minimal friction is rather difficult. It requires some parts that you don't normally see on robots to solve issues with its complexity. On the other hand, Butterfly is relatively simple and straight forward, simply requiring the ability to machine identical pieces for either side of the module.

With swerve, you need precise PID control for wheel angle, and fairly complex calculations to figure out how to angle each wheel at any given time. With butterfly, you basically just have two driving modes, tank and mecanum, which are fairly well understood and easy to implement. Then just a little pneumatics to change the drive being used.

From a usability standpoint, being able to shift between butterfly drive trains using pneumatics leaves open more PDP slots for motors elsewhere, while swerve requires those extra turning motors.

All that said, while both drive train types are cool and impressive, I don't think they're really needed on an FRC robot to do well.