Ok guys, this is a fun one for me! As for the question about wingtip vorticies, the answer is that for the type of flying that we do, it doesn't really matter!

I have been flying full scale airplanes commercially for about 10 years now, and have flown model aircraft for about 10 years longer!!! Also, I completed 3 years of Aerospace Engineering before deciding to fly them instead of design them for a living. The only reason I give this info is to show you how I have come to the above conclusion.

Wingtip Vorticies are cause by the relative high pressure of air on the lower surface of the wing meeting the relative low pressure of air on the top of the wing. One of the laws of lift is Bernollis' Principle. His theroy roughly states that as speed increases pressure decreases. As to how it works with a wing, imagine a flat bottom aerofoil. Now picture that 2 air molecules hit the leading edge of the wing at the same momment. Now picture them travelling across the top and bottm of the wing and reaching the trailing edge of the wing at the same exact time. Which one went faster? The answer is the one going across the top of the wing because it had to travel a longer distance in the same amount of time! Insert Mr. Bernollis' set of math, and VOILA, the upper surface moving air is travelling faster, and thus must have lower pressure than that air passing under the wing.
NOW, what exactly does that tell us??? Easy!! One thing about nature is that pressure ALWAYS wants to equalize in the atmosphere! The realitive HIGH pressure under the wings is wanting to meet and equalize the realative LOW pressure ABOVE the wing. This action is what gives a wing 90% of its lift!!!
Now that I have throughly bored you, wingtip vorticies is just that area of air where the pressures can meet! At the wingtip, the HIGH Pressure air below the wing is able to rise and meet the LOW Pressure air on top. This cause a swirling motion of air behind the wing.
The only time that this type of situation can be dangerous is when another airplane flies into your wingtip vorticies!!! The worst scenario that can take place is when the proceeding airplane is Heavy (at max gross weight,) Clean configuration (landing gear and flaps up,) and Slow speed. When the following airplane flies through the vorticies of an airplane operating as above, the results can be DEADLY!!!
Now as for flying models, you should not eer have to worry, unless you are flying at a real airport with real airplanes operating in the near area! The amount of vorticies created by a model are not very signigant considering how most airplane are flown with greater thrust to weight ratios as the full size bretheren! If you get in a situation where the airplane rolls without warning, just apply max power and do what it takes to get the airplane to climb! That's it!

With all the boredom of that stuff out of the way I can answer your question better. Because the nature of Wingtip Vorticies is to descend behind the airplane, the total amount of drag is less than 2% of the total drag on an airplane in flight!!! As someone stated before, a tapered wingtip design has slightly less drag may be true but only to a small amount. The actual reason that a tapered wingtip cause less drag is due to the fact that the smaller edge area is less for that swirling wind to conteract with! This means it just hits the wing with less area! The down side to a tapered wingtip is that it gives you LESS lift than a flat wingtip because of the lower speed of the air passing over that area on a tapered wingtip.

As for the differences in wingtip design for models, there are advantages either way. Want speed, go with tapered. Want Lift go with flat.

Any questions about this, let me know.

Thanks,

Reg