Stats are nice to review and debate, but the best measuring stick for how a QB is performing is My Eye Test.
When trying to compare 2 QBs, it becomes hard because of the different coaching philosophies, players, and play designs.
So, by my eyes, right now,
give me Dak over Purdy, as purely a QB measurement.
Comparing the teams, including the coaching staffs, San Fran has the edge (mostly because they stomped the Cowboys when they played them this season, and the Cowboys have lost 3 in a row to their NFC nemesis).