There are many different makes and types, firstly based on my experience your location in terms of climate will be a major factor. As will the fit.
My experience on this is an ADCO Tyvek cover, the cover ripped on the roof in two spots but they have replaced it under warranty. It is best to get a cover that fits as best you can, if it is too loose it can tear easier from what I read as the wind will "whip" it more. I also put tennis balls with holes in them and put them over the water spouts on the roof so the cover is touching no sharp edges.
In terms of scratching the glass, I've had the cover on for two years and no damage to any part of the TT.
2010 Starcraft Autumn Ridge 186BH
2012 Dodge Ram 1500 Sport, 5.7L Hemi, 3.92 axle.