The Following is Fact:
The purpose of the drive train is to get the manipulator in position to score as many points as possible in the minimum amount of time.
That’s all you need to know. The best drive train is the one that accomplishes this task the best. Period.
It doesn't mater how many wheels, chains, motors, or gears you use. If 2WD will get the job done, use 2WD and use the time/money/resources saved to work on the manipulator.
The Following is my Opinion:
Historically, 6WD with the center wheel lowered has been the fastest system.
If you need to climb a ramp, 6WD is necessary to prevent bottoming-out.
But if the playing field is flat, why bother with 6WD at all? I mean, two of the wheels aren't even touching the ground. So why bother? If the playing field is flat, save yourself some weight and only use 4WD with the wheel axels spaced 12" apart and casters on the front.
Some will argue that swerve, mechanum, omni, and tank treads offer advantages over 6/4WD. But when you consider how many teams consistently win regionals and championships
without these systems it becomes hard to see what the real benefit is.
As for the 25 vs 35 argument...
2006 Our team used #25 in 2006 with the kit frame and it was a COLLOSAL disaster. We couldn't keep the chains in line, or tensioned and they kept falling off.
2007 Deciding we would never use tensioners or #25 chains again
we switched to #35 with movable axels for tensioning. This dive train was very robust, but also very heavy.
2008 Deciding that maybe we had been too rough on #25 we switched back, implementing the same movable-axel tensioning system we used in 2007. Worked great. Much lighter and we never lost a chain.
So it really comes down to tensioning. #25 needs to be really tight, so you need a good tensioning system. #35 can be much looser. Some teams like 766 and 330 have gotten away without using any tensioners.