Just curious, do you have the means to press the drivers onto the shaft yourself or will you have to pay to have them pressed on? I swapped two sleds to 8 cog drivers this fall, and I don't have the means to press the drivers on so I bought axles with 8 cog drivers already on them. I found 2 axles with 4 drivers on each axle for $40 each, and I sold the take-off axles w/ 9 cog drivers for $45 each. I'm pretty sure that all wedge and gen 2 chassis used the same length drive shaft.
8 cog drivers are more prone to ratcheting if you don't keep the track tight enough. Personall, I would go with 4 drivers if you're not going to use the extroverted drivers.