What is it that is so hard to understand?
If you run or plan to run the stock Slash class, then your golden. Your in for $200.00+ bucks and all is good.
If you choose the SCT class, then the SC10 is less expensive. Your not paying for electronics that won't get used. For the SCT class both the Slash and the Slash will need a ESC, motor, servo, and radio system so the electronics are a wash. But the Slash will probably(we won't know for sure until the Slash and SC10 actually get run together) need an aftermarket chassis which is an additional expense the SC10 will not have to incur.
So again, the SC10 is less expensive IF
you plan to run the SCT class.
The whole money argument is moot once you move beyond the stock Slash class.
I think the sport is big enough to support both classes.