At some of the larger tech companies the salaries for scientists/researchers is usually higher than for engineers specifically because of the extra educational requirements. Where I work there are actually on two separate pay scales to account for this. I can't say this is true everywhere but there are definitely cases where you could have made more had you stayed in industry (emphasis on "could", it's not guaranteed).
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.
