There's definitely variation from company to company, but the gap isn't huge. Let's say the private research role got me $30k more than engineering. It would take at least 16 years to make up for the 5-6 I spend for a phd. Lots of variables though. I made the right choice for myself.
People usually work for 30 years... Grad school is definitely a long term investment but it's not always the best choice. As you said, in the end you have to do what's right for you.
