How's it invalid? So what he doesn't hit cleanup, it's another reason you shouldn't use RBI to evaluate two players on different teams. Especially in cases when they hit in different parts of the lineup, in different ballparks, with different talent levels surrounding them.
You can still evaluate the players though...
They are actually a lot closer than people think. Very similar. Sandoval has the edge in AVG, but Fielder gets him in SLG, OPS+ and a few of the other power numbers.
Fielder: .284/.381/.550, 140 OPS+
Sandoval: .333/.381/.543, 137 OPS+
I'd say Fielder right now because i know that's what he's going to give me. Right now, you aren't sure about Sandoval. that's why this question is a little tough, Pablo doesn't have much MLB time, we're working off 194 games of sample size... who knows if he's capable of taking that next step up, or could have a slump once teams figure him out.
With that being said, I'll take Fielder right now.