Good technical answers from The Shadow.
But I just want to address the very basics here.
Wing fences and winglets serve two very *different* functions.
Wing fences are used to inhibit the propagation of a stall--just as The Shadow described, they help to stop the burbling and airflow separation that begins at the wing root and spreads towards the wingtips during incipient stalls.
The basic idea here is to protect the effectiveness of the ailerons during very slow speed regimes.
Winglets, on the other hand, (again, credit to The Shadow) exploit *high speed* aerodynamics exactly as he described.
So just to be perfectly clear, one would never choose between wing fences or winglets. They each address very specific aerodynamic problems.
By the way, if you're interested in aerodynamics you might also be interested in other "fixes" like vortex generators and slotted leading edge devices.
Try a search first, but if you have any questions, fire away!
B727 Wing Fence
B738 Winglet
