If i was only going to get one though.....neck turning or annealing.....?
They are inter-related.
We neck turn to keep neck tension as close to the same from case to case... As you fire that case, it work hardens and that happens in a couple of firings. Now the neck tension will vary regardless of the thickness. This is where proper annealing comes in... maintain the ductility of the brass as evenly as possible.
And why doing this step inconsistently can drive you bonkers as it can create even more variations in neck tension.
One top US shooter once told me, he never anneals as he just wasn't doing it well.... years before all these fancy annealers existed. He just sold off his cases after 3 to 4 firings... there were no shortage of shooters waiting to buy his stuff.
If I had to choose only 1, I would outside neck turn... but have a large quantity of brass to draw from until I could get an annealer.
Then there is a quality milligram scale, and good dies, and good brass/prep, and good primers and good bullets and good barrels and good stocks with proper bedding, and on it goes. There is not one thing that makes this process better... you are building a system and it has to work in harmony and in balance if you want peak results.
What type of chronograph are you using? If you are using a simple unit, the output may have an error larger then the results you want to monitor. Again, better gear.. more money. But a target at longer distance doesn't lie. If the groups are small and flat, you have all the data you will ever need.
Jerry