Loretta's attributes (.300 + avg, around 20 homers) place him far above the average 2nd baseman (.275, 11 hr). Add to that, at age 33, his consistency is well established, so you know what you are getting.
Halladay is coming off injury, plays for a 2nd division club and will most likely offer you 15-17 wins, 160-175 K's, 3.75-3.80 ERA. These are good numbers, he will not suffer a Hideo Nomo meltdown, but the injury risk is real and lower ranked pitchers can offer comparable numbers (Burnett, Carpenter, Escobar).
Mags was one of my favorites prior to the injury. If healthy, he can produce 35 Hrs, 110 RBI's, decent average. Still, the injury risk is great and there are many across-the-board outfield performers who will still be available.
Loretta provides you with a substantial advantage over other owners saddled with D'Angelo Jimenez-type players who spit out 0 for 5 game after game. Second and third tiered pitchers/outfielders can be had later and more cheaply. Loretta is the best value.