In reference to defense, I think he'll be better in chicago. Keep in mind last season was his first in Houston and they have a rather weird CF with the hill and everything. In Colorado he ranged from barely able to play CF to close to average. Looking at his UZR numbers the big hang up in bad years was range. Incidentally, that was also his issue last year. Houston is 362, 435, 373 feet in left center, dead center and right center respectively. Wrigley is 368, 400, 368. Colorado is 390, 415, 375. So, from a range perspective you would think of Wrigley being less ground to cover.
Forgot another point i was going to make. While it is kind of surprising to see Valbuena gone before Castillo it does leave them with the interesting aspect of having another decent trade piece to add something else. Wood and Castillo for instance should be enough to get you a half decent starter some where.