Hmm... Erstad's stats have dipped in the second half every year of his career, sometimes dramatically, sometimes just by a bit. It's hard to go against a streak like that. I think he'll still be solid, but his numbers should drop a little. If you can get a fair deal for him, dealing him seems to be the safe choice.
Sweeney's second-half struggles are a more recent phenomenon, and don't occur as consistently. I wouldn't worry too much about him. He seems to be in good shape; his recent injury was the result of being hit by a pitch, not being in poor shape. Any player can have a poor second half regardless of their history, but I don't see any indications that the odds of this are greater for Sweeney than for other players.