First off, great work, the icon design is sleek and modern.
Typically I like the look of flat color logos like your #53 (so we could use Pantones) rather than 3D style with lots of gradients and highlights. That said, there is no denying that your #51 design looks slick.
As for color selection, the reason I have been asking for blues and greens so far is that these colors really have that fresh and clean feeling. What color are you using for #53...it's close to black, but is it black? I have a Pantone guide if you want to give me the number.
Here's a few suggestions.
Could we try #53 as is, but with a silver "S" in the upper icon. Also could you try a second version of #51 with a blue "Q" in the upper icon.
Thanks again
Alan