They're great games so I'd recommend buying/renting the God of War collection if you can.
As far as story is concerned, you can't go wrong playing both, but I think God of War 2 would be the more important of the two for it's story. There's a slight recap as you play through and the start of God of War 3 picks up exactly where God of War 2 leaves off.
*edit* Still, I'd highly recommend trying to play through the first two games first.







