Okay..!! See.. Jobs in Petroleum Engineering will be around for a long long time, and coming out of college, and in the long run, there is more money to be earned. Petroleum engineering also allows you to work all over the world and meet many different people.
But, do what you're passionate about. You will not gain anything if you are stuck in a job you hate- even if you make a bunch of money.
You can do PetroE with a MechE degree because if you don't like the oil industry, Mechanical engineering will help you get other jobs(automotive, aero, etc)
THE MOST IMPORTANT THING IS YOUR INTERNSHIP EXPERIENCE, NOT YOUR DEGREE OR GPA.
Regarding Salary, I don't think there is a salary difference. The same position will pay the same amount regardless of the degree. Right now the oil industry is in a state of flux. With oil prices being so "'low", the industry is being cautions. Entry level positions could be less abundant for the next year or so.
So, make your choice wisely.