Swerve:
- Harder to program, but more complex motion is achievable
- Easier to drive? it depends on a few factors a) how responsive the system is to sudden changes in direction and b) how it is set up to drive (field centric or not, depends on the driver which would be preferred)
- More complex machining
- More resource heavy
- Requires more of a commitment, if it falls through you are pretty much back to square one or you are rushing to find omniwheels to fit one end of your robot as you lock the steering on all the modules (trust me, not fun, we did that once, we don't talk much about that year)
Butterfly:
- easier to program
- could be easier to drive if your drivers are used to traditional skid steer operation
- not as good traction in all directions
- simpler to machine and assemble
- generally cheaper because of the lack of need for specialty parts
- if omnidirectional portion doesn't work out the robot can still easily be driven as a basic skid without major code modification or mechanical modifications
- for a team without experience with either system it would seem to me that a butterfly/octocanum system would be easier and quicker to design and build
- possibility of 2 speeds without any gear change
That seems to be everything I can think of, pros and cons, for both systems. I too would explore octocanum before butterfly for simplicity reasons, but I would say it all depends on what your base is (pun intended), if you have experience working with swerve that is probably easier, experience with mechanum makes octocanum easier and experience with skid leans in the direction of butterfly drive so the long and short of it comes down to what do you feel like taking on.
As far as which one of these will get you the most competitive advantage, well I would say that realistically a basic skid gives you the best competitive advantage because it generally leaves the most room for the advancement of other systems on board and it usually results in the most driver practice which is absolutely key. As a side note, from what I can tell the highest scores of any season, except 2015, were put up by an alliance of robots that did not have omnidirectional drive capability.