uh.. winglets basicly increase Aspect Ratio and Area with
the aerodynamic & structural pro's and con's that go with that.
Wing fences don't do that, they just reduce lateral flow of air
along the wing.
As mentioned, winglets also increase stability around the vertical
axis which means a bit more rudder might be required in e.g. a
decrab on landing. (which might reduce max xwind limit on some types)
M