I guess the thing you have to consider is where are you in the standings? If you are still in contention, you have to keep Dice becausethose guys are really only going ot get the call in Sept (for Davis), and more than likely 2010 for Feliz. With that being said though, there are two reasons why this deal is more nuanced than one would think:
1. Feliz pitches in an extreme hitters park and that will skew his numbers somewhat, despite his obvious talent.
2. Wade Davis pitches in the toughest division in baseball. 30% of his starts in any one year will be against the Red Sox and Yankees, which can be murderous against any pichers, let alone young ones.
However, on the (+) plus of the equation, Feliz is a Top 5 prospect in all of baseball, so if anyone can succeed in Texas, it's him. Also, if oyu look at the way TB deployed Price last year, they really limited his exposure at the major league level, which gave him a competitive edge to hitters looking for every clue as to how he would approach them. They will probably do the same thing with Davis this year, and unleash him after rosters expand, provided TB is in the pennant race, which I believe they will be.
One thought though, I would try to switch out either one of these guys for a hitting prospect, which have less injury risk.