I cast my vote for including upped/shard decks here. Doesn't matter how veteran you are, bronze is almost always fastest for getting spins, unless your deck really knocks the socks off of gold.
A little bit of relevant math:
If a deck has 70% chance of winning gold (which is pretty freaking good), the chance of winning 3 times in a row and getting special spin is:
.8 * .8 * .8 = .343 = just over 1/3 chance.
If a deck has 90% chance of winning bronze (which is pretty mediocre, to a veteran), the chance of winning 5 times in a row and getting the spin is:
.9 * .9 * .9 * .9 * .9 = .59 = well over 1/2 chance.
Gold has the advantage that fewer rounds are needed, however, each of those rounds tends to be a lot longer (pretty rare to see a 200hp in bronze).
Plus, if you're doing a long-term sort of grind, bronze is a lot easier on your sanity! (Don't supernova me, bro!)
Tl;dr: veterans have a huge incentive to farm the hell out of bronze and silver, so please do share those upped/sharded/nymph decks