You’ve probably been asked this many times!
I have an indoor trainer that measures my power but not on my bike for outdoors.
I’m tempted to buy a power meter although it would cost nearly as much as my road bike or invest in a coach and use my heart monitor instead.
If you could only afford one, is your money better spent on a power meter or a paying for a coach.
Buy a used g3 powetap wheel. That is a cheap way to get into the power world. It is not light but it does the job! Unless you have a $400 road bike it will not be more expensive.
Ed K
I would say a coach everytime hands down. Simply to have a professional relationship with an expert who has accomplished what you are trying to is enormous.
Not only that but to have a pro giving you advice on timing, which races to target, which ones to not - big picture decisions that have huge benefits is the value of a coach.
A good coach can help you improve using the fundamentals of sports cycling performance. A powermeter is great too but a real person trumps the data the powermeter generates 
Also a Stages brand new left crank is $299 (before your FasCat discount)
Power meter changed the way I ride outside. I also have a smart trainer that has built-in power meter. I like to have the option of taking the training outdoors sometimes. Get a power meter! You won’t regret.
Fascat plans and power meter will make you crazy strong. From there you can assess your goals and see if a coach is needed.
My vote is for a coach. I tried to self-coach and use other plans for 4 years, having a coach was the best decision I made!
I’m new here, but having been coached for the last 18 months I can say honestly, a coach will make you faster, no doubt.
That said, I couldn’t imagine being coached without a power meter, for his sake and my own. It is very rewarding crushing power PRs in trainingpeaks for 10, 20, and 60 minute intervals. I set all my all-time power PRs as a coached athlete.
I’m back to self-coached for now due to the COVID uncertainty, $200/month buys a lot of power meters. 
Long term the coach is definitely going to be the best thing to get you fast.
Without knowing the type of coaching you’re looking at I’d consider putting the first few months of what you would’ve paid for it towards a price point power meter (like a Stages left side only). Then you’ve got the best of both worlds.