Let me start with I have never physically made either of these, but I have done a number of CAD designs for each.
Swerve:
Pros:
- High traction in any direction (i.e. can push going sideways)
Cons:
- Lot of parts to fail
- Requires more precise alignment (harder without CNC)
- Complicated to program (more sensors and processing required in order to drive straight)
- Uses 8 motors (6 for 3-wheel swerve)
- Heavy without CNC
- Takes longer to tune before driving (multiple feedback controllers necessary)
4-wheel swerve is more stable if pushing matches are a possibility while 3-wheel swerve is lighter.
Octocanum/Butterfly:
Pros:
- Inherently supports two-speeds without a ball or dog shifter
- Easier to use COTS gearboxes as part of design
- Easy to program (viable without any feedback controllers)
Cons:
- Easier to be pushed
- Doesn't like uneven terrain
That's just what I can think of off the top of my head, I'm sure other people will have more.