Hey,

The pay in petroleum is nice, but honestly, after about five years experience with ANY engineering profession, if you're wanting more money, you're just a greedy slob. Choice has to be made based on your interest.  Aerospace engineers CAN work as mechanical engineers, and can work very well. There will always be planes and rockets that need to be designed and built. Obviously mechanical engineers will always be able to find a job somewhere, even if we regress to the dark ages. Petroleum engineers, I guess it really depends on whether we as a species decide to move beyond oil like we need to.
Rest it is upto you what interest you more!