You mentioned that you thought about tile. I saw this floor in a magazine and knew right away that I wanted it in my newly remodeled 60 year old house. It combines tile and hardwood for a warm feeling but not the same as the rest of the house that has dark hardwoods. It was not difficult to do either. My builder and I did this one. Here is a link that might be useful:

If sunlight will dwell on the tile for a portion of the day, the darker one will collect and retain heat better--more useful for passive solar--free heat! Even in summer the dark color will take in the heat and tame the air heatup and will release the heat at night when the air is cooler--to even out the day/night disparity in summer rooms. (Plus you should shade those south-facing windows in summer--honeycomb shades? landscaping? overhang? etc.) And in summer you can put a seasonal rug over the dark tile to cut down on heat retention if the floor seems overly warm in places where sunlight touches it.

I would also add rock wool insulation.

Gray flooring is very trendy - which means it will become the dreaded "dated". While light gray as a neutral will always be useful, the extreme over use of gray will turn on itself - just as over use of oak did. If this was a cheap laminate gray floor with a limited lifespan - no big deal (except the waste in the landfill) but you had better love this expensive floor 10 years from now when the HGTV crowd will say "yuck - look at all that gray" - just like they say "Yuck, look at all that oak" now. If you will love it, go ahead. I have my oak kitchen which I like just fine. However, a natural wood floor will add color and will go with just about any style thrown at it. It is best if one applies the trendy colors to easily changed surfaces - like paint for walls, decor, curtains, upholstery and even a backsplash if it is not a huge expense. Listen to SJ - she is a flooring expert. The gray, gray rooms just looked washed out - especially when all the new design pics I am seeing are showing color.