Solder is a converted TE still learning OT, needs a lot of coaching up on technique (footwork, hands, bending at waist), needs more strength in lower and upper body. In short, he is too much of a project IMO, with too much bust poential that goes along with that.
Castonzo is much more polished and NFL ready. Performed very well at the combine also. Some scouts said was the best they have seen in technique and going through the drills since Joe Thomas.
I think others here may feel the same about Solder as myself, as he is still available in the other mock and pick 32 is up right now.