When I farm, I usually farm Gold. If you've got a good Arena farming deck, then you have a high win rate, the reward/cost ratio is high, and you end up getting special spins and therefore winning rare upgraded cards relatively often.
With Platinum, however, you're far less likely to win. I've seen more than one person saying that a good win rate in Platinum is around 40-50%. That's not terribly high. The cost/reward ratio is higher, of course, but it's not
that much higher than Gold (248
compared to 161
at the moment). And a low win ratio means that although you only need 3 wins in a row to get a special spin you're actually less likely to achieve this than you are to get one in Gold, for exactly the same reward.
So why bother? Can anyone who farms Platinum explain it to me?