Billet Upper and Lower recomendations

[Klem]

Do you think there is a harmonic advantage to having the hand rail connected to the upper? Or is it really just for the badass factor?

The only advantage I can think of with a monorail/receiver is you can push the sight mount forward above the barrel with confidence. No requirement to use cantilever sight mounts and also frees-up rear rail space for both NVD mounts and BUIS. If you use reflex sights (0 magnification with no fixed sight relief) they can sit as forward as you like. I doubt if many Grendel users use reflex sights however.

If they allow for a stronger barrel/receiver bind then that is also a good thing.

Looking 'badass' implies use of the gun as an identity prosthetic. It's a tool, a machine. If what the machine looks like (form) is important to you then each to their own, but hopefully form doesn't come at the expense of function.