Several years ago I ran some tests on wing tips with a friend of mine. He had built a makeshift wind tunnel and we used a wing with replaceable tips, We had thread CA'ed to the wing to check for out flow which would indicate the size of the vortex. The results agreed with my flying experience.

A big Round wing tip like on many pre-WW II planes was the worst. The Stick style was next. A rounded block was better. End plates were the best with flat wing ends coming in second best. Other than an end plate, the more you put out there, the worse it gets. Leave them square and flat. End plates will increase your roll rate and decrease your stall and landing speed.