There've been quite a few rumours on the topic since last summer; just do a Bing search on HTC M9 Windows, and you'll find a number of articles suggesting that HTC will indeed continue have Android <and> Windows both.
Had played with an M8 in the VZW store in December, and really liked it; as-is, it's notably faster than my Lumia 928 (Quad core vs. Dual), very nice screen, aluminum body and really good sound. While the camera isn't as good as the 928's, it does have some advantages in speed and interesting functions over the Lumia. This being said, I decided to wait a few months to use my upgrade on the M9 or whatever other high end mobile device (aka phone) comes out in the spring. Better cameras, speed and expandability are on the horizon!