I'd prefer to refrain from making too bold a statement about one idea or another or one design feature or another but I can't help but share my opinion on certain things. It's a process of informed trial and error. You get the best information you can, you speculate on how it fits in to what your trying to do and then you come up with an idea and incorporate it all into a working model.

It's so important though for folks to realize that knowledge is dynamic, what was accepted as the best practice yesterday may not be today. Does it mean that yesterday's practices were wrong? No, it just means that's the best they could do yesterday. There are alot of design considerations when building anything and carefully selecting one piece won't make the machine unless you carefully select them all.

Like I said before there is close to didly squat out there for info on Vawt's except pictures and testimonials from the builders. My advise to new builders is to have a look at what others have done, learn all you can about how the thing works (without an idea of how it works you likely will not have much success building one) come up with a design and build a scaled model first and be prepared to have to do it again. The other option is to as close as possible copy a commercial design that has been studied well such as the Sandia Savonius of the Darrieus or an H rotor or accept lower efficiency and build a drag bucket Savonius. Even if you do though you are still going to have to learn all you can about how each part fits in to the system.

You'll also have to realize that you need to be carefull not to pick a piece off one design then pick a piece off another and frankenstien it all together without first having a good idea what might happen as a result. Vawt's can take many more approaches to producing power than a Hawt so you need to understand that a design feature is likely very specific to the machine it was designed for an may not cross over well into a machine that has a slightly different way of working the wind. Changing the aspect ratio of the machine and nothing else isn't likely a good idea either without considering what may happen or what other pieces may need to be changed as a consiquence.

In all I say experiment, have fun with it and if you need a reliable power producing machine and don't have the time to research and experiment, build one of the tried and tested and very reliable designs such as the 55 gallon drum units.

I doubt anyone can say for sure exactly what effect a certain airfoil will have without trying it and testing it. Look at what's happening, how the machine works, what it needs to make it work well or better and don't forget it spins, sometimes faster than the wind sometimes slower. Airflow over the blades will reverse direction on the down wind side at some point depending on the design and the power producing portion of the unit will also shift according to rpm as the appearant wind direction changes.

Good luck