I have a raptor 6 and 8, and have used both cans on both a 20" 6 ARC and a 19" .243AI.
Not really much specific back to back comparison, but enough shooting to give some of my impressions.
On both cans, my opinion is that the reflex improves tone significantly, which lowers perceived sound, but actual dB reduction is small vs flush mount. I don't have a way to measure dB with enough precision to be useful.
I would characterize them as follows: On the 6 ARC, at the shooter, not a ton of difference to my ear. At the spotter, 6 is nice but 8 is ridiculous. There are times on a busy range I'll be watching the target and see an impact but not register that it was my kid that shot. Recoil reduction is great with both.
On the .243 I notice more daylight between the two, especially with the flush mount. I would still not have any issues taking a few to several shots without ear pro with the 6, but it's a noticeable difference in perceived sound for me from shooter's ear. I have not sat behind them both on that rifle as a spotter so I can't speak to anything but shooter's position. The 8 gets the nod from me on that rifle. Again, I choose to use the reflex all the time despite the weight penalty.
If you put a blindfold on me and rifle in my hands, I probably couldn't reliably tell you which can was on it from the way the rifle feels.
If I shot them blind back to back, I could definitely tell you which was which on either rifle. One shot so there is no comparison? On the .243AI I think I could tell you which one is on there. 6 ARC, probably not with very high confidence.
I would almost certainly recommend the 8 if someone was asking for a first can with maximum versatility on a wide range of cartridges. I have no qualms about recommending the 6 for your stated use, though I suspect once you shoot it with the reflex you will leave it on. This makes it a bit heavier and no quieter than an 8 with a flush mount (probably a bit louder) but to my ear a little louder with better tone, so a wash. It's a little shorter though, which is nice if you're shooting something with barrel length on the edge of too long.
I'd say you really can't go wrong with either one, they are both great cans.