Beyond the "it depends on the kingdom" part of it (which is very true in this case. I would argue even more do, "depends on your strategy", here are some thoughts.
Think about your money density. Let's ignore strategy, other cards, and +buy needs.
Let's assume you have 20 cards in your deck (7 coppers, 3 estates, 6 silver (or equivalent), 4 golds. Total value is: 7+0+12+12=31. So average value of a card is 31/20 or 1.55.
If you add a Platinum: 36/21, 1.71
If you add a gm: 33/20, 1.65
If you add 2 gms: 35/20, 1.75
So, for a pure money play, given that deck composition, 2 gm > 1 plat > 1 gm
That calculation changes as deck composition changes (you really should be optimizing for next shuffle, or maybe the shuffle after that)
You are unlikely to be making these calculations in mid-game though. So you can either make calculations between games, or play a lot of games and start to develop intuition (before I started writing this I guessed which order it would come up, but I'm not sure I would have had the magnitude right. (I would have guessed plat>3 peddlers. In this example that's true, but barely!)
There is an article somewhere talking about money density. It's a good place to start when considering "which is better"
Ed
(check out stopovertravel.com)