My fairly novice opinions:
First, if you look at the pic that cb56 posted with the Mill and Cirrus side by side, you can see immediately why the Cirrus feels longer. The top horn on the Mill extends to the 11 - 12th fret. The Cirrus top horn extends to the 13- 14th fret. Thus, when they hang on you the first fret is about 1 fret width further away from your body. My Cirrus definitely feels like a slightly longer reach to the first fret. I often here people refer to this and chalk it up to being 35" scale. As the picture shows, I don't think the scale is the culprit. It's the ergnomics of the bass' body. The Mill is also 35" scale and still an easier reach to the first fret.
I have a Mill Plus 5 (the beauty that MNAirhead mentioned). And I have natural maple (pau ferro board) Cirrus 5 which I have installed a Millennium preamp in. And I have a ruby red maple (birdseye maple board) Cirrus 6, also with a Mill pre.
I would describe the necks on the Cirrus (both) as being flatter on the back, than the Mill. Thinner in the middle, but thicker towards the edges. The Mill neck is more of a C or D shape than the Cirrus neck. Thicker in the middle, but rounding off to be a little thinner out at the edges. I like them both, but they are different in feeling. The Mill neck feels very much, to me, like a US Lakland 5-string neck. The Cirrus neck does not feel like any other neck I have experienced.
I thought both Cirrus were significantly improved by the Mill preamp. Even when set flat, I think they sound better, but it's been long enough since I swapped them out that I can't really begin to characterize the difference. I just know I liked it better. I liked the Cirrus 5 w/Mill better than the Cirrus 6 w/o. Then I upgraded the Cirrus 6 and like it equally well now.
I definitely agree with whoever said that they can't imagine not being able to get enough booty out of a Cirrus. That's what the bass knob is for. And the mid-sweep and mid knob. If whoever originally said that the Cirrus doesn't have enough booty was in the camp of leaving the bass EQ flat and doing it all on the amp, well, whatever. That's like complaining that your Porsche isn't fast enough, but you don't want to rev the engine above halfway to redline. If you're not going to use all it offers, don't complain about the part you are using.
I have DR stainless taper core strings on the Mill and Cirrus nickel rounds on my Cirrus. I tried Cirrus strings on the Mill, but like the stainless better. I have only tried Cirrus strings on the Cirrus. I really like them and have no desire for anything different.
The sound of the Mill vs Cirrus is just different. Not hugely different, by any stretch, just different. I think it's mainly the difference in J/MM config versus dual soapbars. The Mill offers some growl that the Cirrus doesn't. The Cirrus offers an extremely clean sound that the Mill doesn't. I think the strings I am using accentuate the positive aspects of what each bass offers. Personally, I usually want to hear a bit more growl or cut. I think that's why the 2 basses that I pick up way more than any of my others are my Mill and my Fender Precision.
I'm going to put my Cirrus 5 up for sale soon, but that is in part because I have the Cirrus 6, which offers everything the 5 does plus one more string, and is a color I like better. I definitely think the Cirrus and Millennium are different enough to merit having one of each, if you can afford it.
All that said, if I had to cut back to only having one bass, period, I think I would let the other 10 go and just keep the Millennium. I would even sell my dream bass - my '66 Precision - first. Nice '66 Precisions are expensive, but not hard to find. I have never seen another Millennium that I liked as much as my Millennium. Actually, I have never seen another BASS that I like as much as my Millennium.