I can only answer in terms of my personal experience. I have only compared my RB250 to my 12" AnnaLog NAS arm.I can only recall hearing one RB300 at a dealer years ago. My RB250 is quite fully tweaked... TWL's tweak (search here on A'Gon), the HeavyWeight and new stub, and Incognito wiring of course.
I have not heard the silver in the Nott because it arrived with a tag detached, and I detached a second tag trying to fix the first, so I sent it to Britaudio immediately. I am very satisfied with the Incognito in both arms.
My Rb250 sounds good, but my 294 kills it. I would be tempted to get a 9" NAS arm for the Interspace, but would have to a lot of disposable income and I don't.