Should I build something fun or build something that solves a problem?

Hello, so I'm facing a conundrum right now.

I have the option of either picking something fun to build with no real-world application (but uses cool technology that I want to use). Will have lots of passion this route

or

I have the option of building something that solves a transparent pain point with a group of talented engineers I enjoy solving problems with. Tech is not cutting edge, but it's fun and enjoyable to work with. Medium passion levels.

I was wondering what was the better marker ? Should one spend time solving problems -or- Should one spend time building personally interesting projects ?