I am interested in everyone's opinion as well. There's a bit more to this though. The original (soft) offer came from the team with Renteria (team A). Both A and B (with Garciaparra) lead their divisions and should meet for the league championship. At this point, both would be favored to win their divisions next year as well. A has more hitting this year than B. B has been reluctant to make any deals that strengthen A and weaken B for this year, given the chances that they could meet in the playoffs.
Garciaparra for this year:
.310 .352 .528
Renteria:
.305 .364 .439
Garciaparra for next year:
.325 .362 .553
Renteria:
.323 .376 .477
There is also the long-term value of both (and Brown), for which I think there would be a lot of different opinions.