With SPG's double stays promotion running right now, it only takes 13 stays to get Platinum.
I think a lot of the decision would depend on your travel patterns. Where do you travel? What type of hotels do you like to stay in? What price ranges? How long is a typical stay? How many nights and stays do you have in a typical year? How important are upgrades to you? Do you mind moving hotels (perhaps several times) during a stay to maximize rewards and stay credits?