Hey, glad to hear from you again!
Here goes. I have owned both and with a plethora of gear in and out on both. Both seem to have their own attributes as well as deficiencies.
Revelator tweeter - Precise imaging, laser pin pinpointable if you will. Especially in a cabinet with little resonances and a good crossover. Responds well to cable and upstream upgrades. A tad bright when pushed toward the upper limits but extremely good reproduction at low to reasonable levels. It is only when pushed hard does it get a tad "sizzly" at the upper end of the volume knob. I can not say enough about the imaging, almost surreal at times. The natural roll off and reproductive efforts are simply top notch. Basically, it could be considered by some to be a world class tweeter.
Millennium tweeter - Great all around tweeter for all genres of music. Can be pushed harder than the Revelator without the bright top end at the extreme volume level. Just a tad more relaxed toward the top end of the spectrum. Comparable imaging to the Revelator but in my experience, only with upstream gear that is more toward the upper scale. The lower the gear, the worse it seems to be but with the right gear and cables, can rival (not top) the Revelator. The natural roll off and reproductive efforts are simply top notch. This tweeter seems to be a little more forgiving of recording deficiencies, can get louder without fatigue but still has all of the subtle nuances needed to be a world class tweeter and you still don't need to strain to hear them. Also responds well to cable and upstream upgrades.
IMO, both are a top performer, just depends on your preference. The aforementioned are the main differences between the two. If you have a specific area you are a little more concerned with, let me know. I will offer you what I have observed and clarify a tad bit more. Hope this helps.
In search of accurate reproduction of music. Real sound is my reference and while perfection may not be attainable? If I chase it, I might just catch excellence.
The best way to enjoy digital music reproduction is to never listen to a good analogue reproduction.