I'd play safe and go with Kearns. With him, you pretty much know what you'll be getting, while Sheffield could remain on the shelf or put in a few more of those pinch-hitting appearances all fantasy owners love.
Even if it were certain that they'd both play the same number of games this week, I'd give Kearns the nod. Hitters can take a while to get over hamstring trouble and return to full effectiveness. Hopefully, both these guys will do well, but I'd play this one safe.