Hi Nico,
You're NOT the first photographer who has grappled with this decision.
I too shot an M body on film (M6), have a 5DmkII, and an X100. So, that's where I'm coming from recently.
I didn't like the X-Pro 1 originally...I played with it for about five minutes in a store. Then a friend of mine bought one. I'm a prime shooter, and he was out with his XP1 and 35/1.4. I was out with my 5DmkII and 50/1.4...so, similar rigs.
His X-Pro 1 grew on me tremendously, especially since I'm already an X100 owner and know how it operates.
My want list was a Leica M-sized body and multiple focal length primes.
My plan this year was to buy a Leica M-E or M9. BUT...the price of admission, besides being patently ridiculous, would make me treat the camera as something more precious than a camera. I'd be so hesitant to travel to non-first world areas, I'd be hesitant about weather, accidents...in short, it was too much money tied up in a small electronic device.
I just purchased an X-Pro 1 body, used, and I'll take delivery next week. A lot of depreciation is gone from the price already, so much like my used M6, I'm much more willing to shoot it as a tool, and not treat it like a diamond necklace, to use when conditions are perfect.
My last thought is this: The M-E and M9, and to much extent the M-Monochrom, are old technology. There is nothing wrong with this. However, to pay such an admission price to get older technology is crazy in my opinion. Even the new M is finally catching up other current cameras in technology, but moves away from it's purist roots. I think the M body heyday was the film Ms. Now, the Leica crown jewels are just the lenses.
Leica Ms are like Ferraris...a terrific and storied reputation, but in it's current form has very little in common with traditional Ferraris of old. The X-Pro 1 is like the Nissan GT-R...filled with technology, cutting edge, similar performance, and one-third the price (but still exotic-enough).
I can't push either one to it's limits...they're both better than my driving will allow...so am I missing anything?
The Fuji image quality is terrific, the handling and OVF are RF-like. Now, it's time for me to quit worrying and go out and make some great pictures.