First there was aluminum windows. Then later the drawbacks of aluminum were discovered. Now we have vinyl. It appears that fibreglass frames will be the next generation.
The window people I've contacted don't seem to be too thrilled with the fibreglass. They say they're currently too expensive for most purposes and claim there is not that much of a technical advantage over the vinyl other than cosmetic.
So I guess it's up to the consumer to decide. But the qestion has to be raised if the fibreglass is truly an advantage over the vinyl as vinyl has over aluminum.