Answer
The half ring has the stronger electric field at its center.
Work Step by Step
If a test charge were placed at the center of the complete ring, it would feel no net force because all the forces cancel out. The force, and field, are zero there.
At the center of the half ring, a positive test charge would be repelled to the right. The electric field at the center of the half ring comes from the superposition of many electric field vectors. Their vertical components cancel, and the horizontal components add to produce a resultant electric field pointing toward the right side.
