I didn't play a Bluthner 6'10 but I played the Grotrian 225. I played the Bluthner 7'8", which might be more the apples to apples comparison to make.
Both pianos were fabulous. Regarding sound, the Grotrian was a powerhouse - the bass was amazing, and there was a reallyt cool sound of attack that I liked, but not everybody does. The Bluthner was less powerful, but had the most beautiful dark rich tone - perfect for Beethoven and Chopin.
Regarding touch, if you like a medium touch you will probably find the Grotrian to be on the heavy side, and the Bluthner on the light side.
Both are incredible pianos and you would be happy with either, I'll bet. Have fun shopping!
Mindy