IMHO, get a smart trainer first, power meter second.

A power meter is only as good as your ability to interpret the data. For most, it's nothing more than a pacing tool, because the "user manual" is Andrew Coggan's 300+ page book.

A smart trainer allows you to use an app like TrainerRoad to hit the ground running. After going through a full training cycle, the numbers will be much more meaningful when you see them out on the road, and Coggan's book will be much easier to read.

As for how these products compare to other bike upgrades, there is no comparison. I would sooner ditch my carbon bikes and revert to my 2008 Cannondale CAAD9 than give up my power meter and smart trainer.