I believe the best advice I ever read on drive trains was a quote in a paper by team 33 that said ~"the best drive train in robotics is the one that gets you to the spot on the field where you want to be as quickly and as reliable as possible."
The Simbots have a terrific resource on the different drive train types including a general weighted table.
http://www.simbotics.org/resources/m...rain-selection
The only addition to the ones mentioned on that page would be the octo./butterfly drive which had been mentioned earlier in the thread.
For butterfly I would start looking here
http://www.teamneutrino.org/seasons/...bot/butterfly/ on team Neutrino's page.
This site has terrific pictures/caddings on the different drive types.
http://www.frc-designs.com/drives.html