Well this isn't a 5 string comparison, it's a 4 string comparison.
I recently purchased a Cirrus that it impeccably setup..it almost plays itself.
The Cirrus and the Millennium are two totally different basses in feel and sound. I have no problem just picking up the Cirrus and playing it, but switching from a Millennium to the Cirrus is quite different feeling. The Cirrus being neckthu has the neck above the body by the depth of the fingerboard. The Millennium sits more above the body (almost twice as much). The Cirrus feels a lot longer. Both are 35 scale, but it you look the Cirrus has a smaller headstock. The fingerboard is about 2 inches longer, and the extension is towards the headstock (My Skjold takes the neck into the body somewhat and doesn't feel as long.) The difference in length is about the distance of the nut to the first fret. The Cirrus neck profile doesn't seem to be something that I can tell is a lot different from my J/MM Millennium (all birdseye). My regular J/J Millennium neck is a bit thinner.
Soundwise they are just quite different...as is my J/J Millennium versus my J/MM Millennium. I think that being neckthru it has a lot more presense in midrange. I might try the preamp mod, but I can get great tones out of the stock preamp, so I doubt I do it.
Overall, I just have to say they are QUITE different. I love the Millennium and the Cirrus...but boy the Cirrus is just real effortless to play.