I've owned all of the SRT series and I prefer the SRT7. Still, the SRT6 is a solid light. As you said, it is a more pocketable and simplified version of the 7.
It isn't the perfect EDC in my opinion, but it has a smaller head than the 7 and has that fancy striking bezel. I find that the absence of the color beams is a negative, but then I see the thought process behind doing so- Nitecore wanted a no-frills SRT light.
If you already own the SRT7, I don't see a compelling reason to have the 6. The difference in head size is .24 inches, so a quarter of an inch- you probably wouldn't notice the difference in your pocket.
Oh and in terms of the SRT ring, the 6 ring feels exactly the same as the 7 ring. This is significant in that the srt3 and srt5 rings feel a bit more rigid and poke out less from the light.