Whoops - looks like this question snuck by us two days ago... I really, really like Pierre, but for this year, I'd go with Damon overall (if speed is your only concern, then take Pierre).
At the moment, Castillo is playing better than Boone - this isn't even close (in roto scoring). The key for the rest of the season - apart from health - is Boone's average. If he can raise it, his value goes up immendely. Still, I'd prefer Castillo at this point, especially if you can afford to lose a bit of power.
If you do suggest a trade such as this, you might actually be able to get a toss-in along with Castillo. In many leagues, he is consistently undervalued. Good luck!