I have both -- in fact they're the two standard Blade 98s that I have in my bag right now. I also have a Blade Pro 16m, but that's a totally different frame...
v6 non-CV: a little stiffer, but not by much; a little bit more access to spin, but not by much
v9: feels lighter to swing (but I have an underspec frame), so needs a bit of lead to match the feel of the v6; same amount if not more power; more stable.
I like both rackets, and have used them both within the last couple weeks. I used to main the v8 but I sold it when I got the v6 non-CV followed by the playtest v9. I think the v8 was the softest and the most laser-precise of all the Blades I've tried. But the v6 and v9 have more pop. Still, none of them have put-away power and they make you play a little more defensive tennis. They're easy to keep points alive and they're remarkably stable. Won't get pushed around too much, either on the baseline or at the net. But I don't think you'll notice a marked difference in stiffness. I think all things considered I prefer the v9 by just a nose because of the extra stability and power.
Favorite strings in them: Element, Pure Rush, Alu Rough, 4G, Wasabi pro.