Probably Walker, even if he does only play 115 games. I'm not sure what the current situation with Holliday is, but if it's a keeper, I think you take him and just wait this year out, because in COL he could be a very good player in a year or two I think.
Better year? Walker if he stays healthy. If not, Holliday may put up better cumulative stats. I suppose they can be viewed as similar, non-complete players: Holliday a good player for the 81 games he plays at Coors and Walker a good payer for however long he can stay healthy. I would rather have Walker.