I can tell you from personal experience that how likely a whistle is to have condensation issues does indeed have a great deal to do with materials used and the construction of the mouthpiece / fipple.
When I first started out, my fipple plugs were made of wood and the fipple themselves of copper. I had many reports and emails that the whistles nearly never - if ever clogged after just a few minutes of playing. The only problem with this was that there were a few problems with consistency of materials with the wooden dowels and I did not have the lathes / mchining equipment to make this a standard so I switched to Delrin (a type of hard plastic) for the fipple plug.
I still had / have very little clogging problems due to the design of the whistle and how the fipple attaches to the body. There are several makers who have taken a similar approach since those days.
One of the biggest reasons you will get condensation issues is that the smaller the airway, and more squared off the window is where the airway comes out to meet the blade, the more of an effect any kind of disturbance in that tiny airstream is going to have before it hits the blade. Such as condensation.
Several people have suggested various soap, detergent, and commercial products to help alleviate this issue and I would highly recommend following any of those of your choice with any whistle that seems to be giving you 'clogging problems'. It is simply the nature of the beast in most cases, but one easily tamed.
Take care and happy whistling!!
John
"Remember... No matter where you go... there you are..."
-Buckaroo Banzai