I'm hunter engineer since the beginning of my main char.
I can say there is no difference on second tradeskill.
The engineering is not the tradeskill you can earn huge cash. Of course, there are some things you can make and sell.
My experience is telling me, that better choice from mining is skinnig. Why? It's simple. If you want ore, you have to go to Un'goro or another area and ride around searchng the veins. This is taking precious time. In this time you can be inside any instance. And running thru UBRS i.e., you can skin a lot of dragonkins.