They are both great engines. The 4 cylinder is just smoother. In the same way that the 5 cylinders in the L3750's and L4150's are smoother than the 4 cylinder in the L3350.
I think its a good price and a great little tough tractor, I luv mine. The 4 cylinder is smoother but I think the 3 cylinder of similar HP has more torque, especially at less than peak rpm.