MERN has picked up a lot in past few years and actively used in startups and small to medium size applications that demands rapid development. Lot of startups are offering very good pay for MERN stack. Having said that, Java is still the preferred choice for enterprise world, large scale applications that require complex business logic. Spring boot rules the java eco system and widely used in all big enterprise companies.
It all comes down to your choice, if ur looking for stable job in service companies, investment banks etc java is always preferred choice. If ur looking for start ups with higher pay, a bit of risk taker, you can opt for MERN.