Building in my case saves $300-400 off the top. That’s ignoring tax and dealer cost.
We won’t count gunsmith rates.
And since the upper is already assembled and headspaced, and I can assemble a lower in 20 minutes including digging out the tools, its a no brainer.
In the case of the 20″ upper, I’d likely swap out the handguard assy and front sight base for an adjustable gas block and a lightweight free float tube, but not until the rest is fully tested and vetted.
Basically I like a couple hundred rounds through the gun before I start making changes and polishing. Shows where the work is needed and which parts are more likely to be problems.
Some guns I prefer to build, some buy.