I think you should have a test ride, it's difficult to properly explain the difference but with the EP8 I barely touched the pedals and it took off like a shot, the 6100 felt much more like riding a normal bike by comparison.
You can but each has more torque than the comparable settings on the 6100. Like I said, if I was carting around massive loads on the regular then I'd probably opt for the 8 but if kiddie carrying is the main use then the 6100 is more than enough, plus there's a pretty big jump in price to the 8.
I think you should have a test ride, it's difficult to properly explain the difference but with the EP8 I barely touched the pedals and it took off like a shot, the 6100 felt much more like riding a normal bike by comparison.