I think it depends on a lot of variables. How do you like your current position and company? Are you looking to replace a company soon? What are you passionate about? Front end? Everything? I would go for both in parallel. Try ngrx for one weekend and think where it can be part of your company solution, see if your boss think so too. Try spring boot. After your tried the surface of both, see what is more interesting for you.
Hope I helped, Good luck!