What Danzo said above is true- that really doesn't matter which card you pick, as reward is decided before you do so. And I'm pretty sure that it does apply to mine cards too. So, you just have some tiny chance of getting essence and just the click count not your pick. As for me- so far I got one purple essence from those but I go for just the free draws.

But I strongly agree that it would be nice to have a chance of some other rewards from these cards, like mentioned Cave Keys, especially that beside one free draw, there are coupons involved and getting ores for them is not really so great :S