Which flap is better suited? Split or Plain?
As usual, the answer is: It depends.

A short comparison would read like this:


Plain flaps have better L/D when deployed at small deflection angles.
Split flaps have a higher maximum lift at the same deflection angle.


Therefore, a plain flap can be used for take-off or slow flight, as well. Split flaps are only good for landing, but in this they are better than plain flaps because the increased drag makes the landing easier.

Airflow on the bottom of the airfoil is very similar, and on the top the plain flap will exhibit flow separation somewhere along the flap chord while the split flap has a well defined separation point at the trailing edge. This should produce a more steady wake and more linear characteristics of the split flap when compared to a plain flap.

Proof: Figure 7 from Chapter 5 of Sighard Hoerner's "Fluid Dynamic Lift":