Should I trash Copper or Silver with Mine?
I'm going to have to disagree with Pat; I hate having Copper in my deck, and rather have a consistent deck of Silvers.  Yes, it's nice if you get Gold - Copper - Copper in your hand, but Silver - Silver - Copper will buy the same amount.  Once you start running out of Coppers (or if you only have a Silver in your hand along with the mine), then you can start upgrading to Golds.  

It's kind of hard to answer this question in vacuum, because there are so many different possibilities that could make one answer better than the other; for example:


If you're running an early Moneylender, upgrade the Silver to Gold, and use Moneylender on the Coppers to really ramp up.
If someone is playing multiple Thiefs, I may not even bother with the Mine.  Or if I do get a mine, I'll upgrade Copper to Silver since losing a Silver isn't as bad as losing a Gold.