I'll give you a criteria according to my experience of working simultaneously with NRG and THIES sensors. I ran a wind prospecting network of over 80 masts of 50 m each, in Cuba, and I installed my self over 50 % of them. One thing that favoured your plastic cover anemometers against the THIES' anodized aluminum body is the lightnings. I had, 'till date, 5 anemometres that were literraly stocked by indirect lightning stroke, the bearing got virtually melted. This is something that naver happended with NRG anemos. But, I've notice that NRG anemos have a bigger inertia than THIES ones, at the moment of start running, like if the bearings of THIES had less friction, and this is something quite interesting, that I've found in a mast in which i installed both systems. 

 Andres, good information, thank you! 

I just wanted to point out that while the Thies may have a slightly lower moment of inertia, the NRG MAX #40 will reach linearity by the time wind speed reaches 4 m/s.  Any speeds lower than this are of little concern with respect to wind energy, because there is essentially not enough energy in the wind for production. 

All the best,
Dave