Well there is another HUGE factor to take into consideration which is analog recording vs digital. All U2 albums were tracked (that means each instrument being recorded with a mic or direct) through analog tape machines. Achtung Baby and POP were no exception but they were then dumped after tracking into a digital workstation to mix them. The beauty of this is you can get the warmth of analog tape and the ease of mixing in a digital computer. All albums prior to those including JT were recorded in all analog including mixing.
Then in 1998 U2 discovered a digtial machine called the RADAR and it is the closest digital has come to capturing the warmth of analog and they used this to record Sweetest Thing.
They liked it so much they used it to record ATYCLB so in other words that album was recorded in complete digital as opposed to all their other albums before. I went to music recording school for 2 years in the early 90's and we used a Studer 2" analog tape machine and trust me the sonic quality of that kind of machine is very difficult to get with ProTools or any digital machine especially when recording distortion.
Digital has gotten a lot better over the past few years but i find that a lot of songs I hear these days are very compressed sounding (as if all the stuff is squished into a tiny rectangular box, and all the space around the rectangular box is empty, leaving us feeling empty) My theory is that analog tape is capturing harmonics that whether we can hear them consciencally or not is just not getting captured by digital yet, which is why so many songs these days leave you feeling empty and just not satisfied. It's like then you eat at McDonalds and you feel hungry one hour later and your like wtf ?!?
Digital right now is at 24bit, 192 khz and 10 years ago when i rented an Alesis ADAT 8 track machine it was at just 16 bit 44khz or 48khz, so things have gone up dramatically and i do think digital will become as good as analog in the next 5-10 years but one of the reasons i don't really like ATYCLB is because listening to many of the songs that i actually like live sound empty on the cd.
The biggest factor in quality is to do with the bits more than the sampling rate. 24 bits compared to 16 is light years ahead but 192khz is not as much of a difference over 96khz but 96khz over 48khz is a big difference. I totally am for mixing digitally for all the reasons you said but when i make my album next year it will be tracked in analog without a doubt and then mixed digitally.
