Fred,
This will be more information than you want.
My test for checking digital digital use to be if I could shoot a full length person in a standard magazine vertical format crop of 8 3/8" x 11" and read the detail in the eyelashes then I was fine.
The original 1ds did that, the next generations of Canons all did that.
Then as pixel count moved up I changed that standard to if I could shoot a full length person in a horizontal format and hold eyelash detail then that was fine.
The original 1ds sometimes got close, my 1ds3's do this well , the 5d2 really does it better, the p21+ does, the p31+ does easily, the aptus 22 did, my Leica M8 uh sometimes, depending on focus.
Then the next thing I looked for was skin tone color.
The Phase's are very color sensitive and see almost every color and tonal change on a face, the Aptus was much nicer on skin, though nearly all faces go through some kind of photoshop work so that's not the most important thing in the world. Actually the Phase backs hold so much color you can go almost anywhere with them, but I've always had to do some kind of paint to make corrections on their sensitivity.
This image is from the p31+
and took a little more correction than this image from a Canon that was shot during the same session. Both were shot with tungsten.
Out of all the cameras I presently own, the best skin tones with studio profoto flash are 1. Leica M8 (which is kind of strange because its not really made for studio flash),
2. my Canon 1ds3, then the brief time I tested a Hasselblad 39 #3. With C-1 V6 skin tones for my phase backs is much better than the previous versions of c-1.
Nikon loaned me D3's that I used for a period that were great for low light though I always fought color, regardless of the color. I found them very difficult to work, unless I was at very high iso. Funny thing is they held detail well and hair would always look sharp, actually gave the impression of more sharpness than was actually there.
I was going to buy d3x's when they came out, but didn't because everything I owned still worked fine and I didn't like the fact that if you tether a D3x I believe the camera lcd goes blank and nearly all the new equipment we now buy is for motion.
I do know a good number of very good photographers that moved from medium format to the d3x's and are more than happy for commercial work, so I assume the D3x is probably the 35mm equivilent to medium format.
__________________________________________
In lenses all my contax 645 Zeiss lenses, 35, 45, 55, 120, 140, 210, hasselblad 110, are all razor sharp, the Canon 85 1.8 (non L) razor sharp, same with the canon 50 1.2 and the 35mm 1.4. Most Nikors are sharp, with the 200 f2, steller, same with the sigma 50mm and with all the leicas, though I rarely if ever use anything but the 28mm on a leica.
On the Red I have a mess of lenses that all have different results. The Zeiss 35mm is beautiful, the Nikon 14 to 70 is almost as sharp
http://ishotit.com/double_page_from_red.jpg and very good for motion as it has very little breathing and no jump as you change focus. I've tested the Zeiss Compact Primes vs the still primes and found no difference in sharpness and speed.
The only thing about the ziess still lenses on the RED is they are tiny and looks kind of funny,
but they work amazingly well are very smooth to track focus and the price is 1/2 of the compact primes.
But all in all most modern equipment is very, very good with few bad lenses or cameras unless you're micro viewing the corners.
As far as any real difference. It's always subject, light, dependent and probably just how I feel the moment I push the button. If I think it's pretty then "that's" the right equipment.
Tomorrow it can all change.
