Google says these are the differences:

Global Entry - Pre-screened, costs $100/5yr and involves in person interview and background check.

Mobile Passport - Not pre-screened, FREE, allows you to bi-pass the declarations kiosks and go directly to border agent screening.

With Global Entry you have to stop at the Global Entry kiosk. Since you still have to show your passport to somebody I don't see the advantage of Global Entry over Mobile Passport. The one time I used it, several years ago, we bypassed all kiosks - walked right up to the border agent and he scanned my phone without even looking at us.