IS IT BETTER BEING A DIESEL OR AUTO MECHANIC? HEAVY EQUIPMENT OR REGULAR AUTOMOBILES?

I would say that having experience in both fields would be most beneficial. There are some differences between the two that are relevant but, speaking from experience, they can be considered negligible. Learn both. Heavy Equipment would probably get you more dollars if you decide that you want to work for a firm that exclusively deals in diesel engines. The most successful techs I know can manage both diesel and auto.

Find your passion and run with it.

Fewer and fewer cars today are being built as diesel. My husband is an auto mechanic and I would recommend regular auto, rather than diesel.