Oh, I would join the Garlean Empire in a heartbeat. No doubt about it.

Think about it, all the city-states of Eorzea are constantly under threat from either the beast tribes, Primals, dragons or internal class warfare. Meanwhile, most of them have so little technology that their living standards are basically medieval.

Meanwhile, the Garlean Empire has eradicated all of these threats, and offers more advanced magitek than any of the other city-states could possibly produce in the next 50 years! And considering most of their forces are from the regions they've conquered, the actual civilians of Garlemald hardly see any combat. This would most likely make Garlemald the safest place to live in all of Eorzea!

Heck, if you strike it rich, you could live in luxury with all the magitek you'd ever want.