"Better" is relative.
The slot is easier to get the CG right (motor is farther forward) and will have better control at lower airspeeds (prop wash is over the tail control surfaces).
The pusher is structurally stronger (no hole in the middle of the airframe), is a bit more flexible to the airframe you're modeling (tubular fuselages aren't as well suited for a slot, but still can be done), and is generally much more scale in looks.