Which 2019 iMac i5 vs i9 for Final Cut Pro X?

All Apple's prices are ridiculous. They charge what they charge simply because they can. Just bear in mind that you're stuck with whatever storage you choose to purchase. No one ever regrets getting too much.

The performance difference between the two CPUs are small. You need to compare the two side-by-side. If you do that, you are certain to notice a difference, but it's up to you to determine if it's worth the additional cost. Again, no one ever regrets buying a more capable CPU.

Configure and use 8 GB to start with. You may determine that it's more than adequate. It's very easy to determine if a Mac's performance is being limited by available memory. Please read Use Activity Monitor on your Mac - Apple Support. Run whatever programs you will typically need to run. If the "Memory Pressure" graph is frequently "red" your Mac will benefit from additional memory. If not, it won't. It's that simple.

You did not ask but the first consideration should be all flash storage: 2 TB.

All other considerations are secondary, although if you must choose between an upgraded CPU or GPU, I'd choose the i9 CPU.

I pay no attention to reviews. They are all equally worthless. Go to an Apple Store and use both, side by side. Decide for yourself. Spend as much time as you need.

FWIW, I agree with John that an SSD is the first thing you should choose. Even a 1TB SSD. The next thing I would choose is the 8GB RAM option then buy at least an additional 24GB of third-party RAM. Then the CPU and then the GPU.

Honestly though, no one can tell you what an addition 10% or 50% or more of an increase in performance is worth to you.

FWIW, if you really want to get into serious video editing/production,

the more cores and horsepower in the CPU the better. Generally, this

impacts video work more so than graphics cards.

Graphics cards are more important for things like 3D rendering and

very complex realtime animation. They can help with photo work,

but only if the app leverages the GPU. Some do and some don't.

The only other thing that graphics cards may impact is the number and

resolution of external displays.