Good question. Carpenter was a plus-fielder at third base this past season, based on statistics from Baseball Info Solutions. In 611 1/3 innings at 1B, Carpenter was a plus-6, Goldschmidt was a plus-7. Carpenter was a 1 Defensive Run Saved, and that ranks 15th at the position. Goldschmidt was plus-6 DRS, and that was sixth.
At third, Carpenter was plus-5 (13th in the MLB at position), and he was a plus-6 DRS (seventh in MLB at third). Gyorko was a plus-6 and a plus-6, respectively.
Here's the way to look at this: Aggregate. The Cardinals didn't have many scoops at first base. They didn't turn many poor throws into outs stolen at first base. Goldschmidt would be an upgrade in that regard, and would give the Cardinals two Gold Glove caliber fielders on the right side of the field. On the left, they would have a larger, better target with a good glove at first, and there should be more shifting that gets them into spots to take outs. The defense has to improve as a group, and what does that? Back to the upgrade word.