When you buy a complete AR, there is more tax than buying it in pieces. The 10% excise tax does not apply to just lowers (stripped or built) since it isnt a complete rifle.
As for what you posted, it looks like the separate has a stealth lower (no markings on magwell) vs a regular roll marked lower. Functionally they are the same.
Its not a sales tax, its the Pittman-Robertson Act. We, as sporting goods buyers, are taxed to fund wildlife restoration at 11% per COMPLETE item. Parts, components etc…are not taxed. All hunting and fishing equipment is taxed BEFORE it reaches the store, but the tax is built into the MSRP.
I did my first build using a stripped lower and a parts kit. I prefer nitride. Sometimes PSA runs free shipping promos on certain components so that helps to make a decision one way or another. Next you could try different caliber uppers to put on your lower.
I really like using a scope like the Vortex Strike Eagle or a Primary Arms which both offer first focal plane 1-8x power. With an illuminated reticle on 1x it’s basically a red dot. On 8x you can easily take shots at 400+ yards. The reticles both offer the ability for battlefield range compensation and windage hold overs. Check out some YouTube videos.