In terms of pure upside, I picked Miles. In terms of talent and freakish skills, he has the ability to replace Posey in everything but three point shooting, but he has skills to make up for that in terms of shot blocking, rebounding, and passing. Plus, at the minimum, he could be the steal of the year.
That said, while I like Walker, I'm also eager to see Giddens play. While I've seen little of him, the scouting report makes him seem tailor made to replace Posey: he can shoot the three, he exerts effort defensively, and he has super long arms. The knock on him seems to be more along the lines of his ability to take people of the dribble and create for himself; however, that is of little consequence on a team with Garnett, Allen, and Pierce. If anything, we don't need players like that, who need plays run for them to score.
While I was certainly hoping for more established players to come knocking down the C's door for a roster spot, I will say I'm cautiously optimistic about this year. It would've been great to see a little more certainty on the roster, but with the Big Three plus Rondo, this team can withstand some growing pains and still win it all.