What skill does Walker possess that Giddens doesn't?
Walker seems to be a significantly better shooter at this point in their development. Last year in the D-league, Walker shot 40% on 3pt shots to Giddens's 28%, and Walker's FT% was 80% compared to Giddens's 68%. And anecdotally, in the summer league games this year, Walker showed a nice step-back mid-range jump shot a la Paul Pierce, while Giddens's game was still largely driving to the basket a la Tony Allen.
This is obviously based on just a small sample, but when you couple it with the fact that Walker is two years younger and that he plays about 2 inches taller than Giddens -- and that the C's brought him up rather than Giddens last year -- it's reasonable to see Walker as the better prospect. (Which isn't to say he's perfect, or that Giddens isn't better in some areas: Walker needs to work on his handle, play defense with more intensity, and rebound a bit better.)