The P25 is quite old now (a generation old) and pretty noisy off base ISO
I think you need to test for yourself
There are physical advantages of the P25l
easy clean sensor
usability on view camera
high flash synch (dependant on camera)
etc
I think you will see little difference on a striaght test in good conditions BUT once you take your canon file and run a few lens correction filters, smarten up the verticals, crop it to a client requested aspect ratio etc it may start to fall apart
MF files seem 'deep' ie you can do stuff in PShop - I think it is more than bit depth
I would however never only own a MFDB - too slow (ISO) so you will need a back up DSLR too - you probably have one !
SMM
A slightly different view.
1DS3 files well exposed crop very nicely to square whilst still retaining an 40MB 8bit Tiff not upsampled, so cropping not really an issue.
Use of a shiftlens negates most straightening issues.
Most good Canon glass eg: my own 35f1.4/50f2.5/85f1.2/135f2,70-200f4 and to a great extent my good sample 24shift require virtually no lens correction filters.
As far as i can see, compared to my old Phase One P20 back, the only thing that the Canon cannot quite match is a certain quality of colour in well lit daylight shots, and i think this is more down to using Lightroom for the Canon files whereas the Phase files were always done in Capture 1. In mixed light or low light longish exposures (not high ISO) the Canon thumps the back.
A P20 is just a cropped P25 so absolutely no difference other than filesize, however a P25+ is a different beast intirely and i believe very much improved colour and long exposure performance.
Gary.