This has been discussed. Try looking under the Designs Thread
, or use the search. Please check those before making a new thread in the future.
But anyway: not much.
Theoretically, tip plates get cleaner airflow, and slightly improve the lift of the stab. But if they're not parallel, they add a ton of drag. Another attribute is that they're not adjustable. This can be both good or bad, since it eliminates any chance of adjusting during flight, but it also means you have to change the angle of your boom if you find you need to change your flight diameter.
I'd say in general more advanced building techniques utilize tip plates, but planes can be made just as good with a single vertical stabilizer.