Help needed with motor set-up

Help,

I am currently working on a project to covert a ARii Mazda RX-7 plastic kit to a slot car, but I've encountered a problem...

I have recently bought a HRS Slot-it Chassis to mount the kit on, but I need a sidewinder motor so I can fit in the plastic kit's interior (or a Fly one), anyhow can someone tell me what on earth is the different between Slot-it's 38/34/32/26 tooth sidewinder crowns?

Does the amount of teeth help with performance or smoother running? And what would be the best for a fairly large home Scalex Sport circuit?

Oh, it's all getting a bit confusing!

Cheers

Mark