If you undestand how a winglet works, then just think of a raked tip as a winglet laid flat.
They're both doing the same job, i.e. increasing the effective wing span to reduce induced drag.
The raked tip is a bit more effective for a given increase in wetted area and it doesn't carry it's load through an angled joint.
If your airplane is span constrained, then a winglet might be a better choice.
You can read about raked tips at the following Web Site (cuttently offline for an update.)
http://www.clarkson.edu/~visser/research/wingtips/