My strong preference is for an M Leica (M3, an M4 if you really insist, or an M6) with modern glass (Hexanon M 50/f2, Zeiss ZM 50.f2, and, off course the Summicon 50/f2). A Leica iiiF with a Canon 50mm/f1.8 or a Nikkor 50mm/f2 might also tempt me (the Canon maybe more so, I was shooting with one of those in 1964 (excuse me if I digress).
That being said, I've recently acquired an x pro 1 (at the current fire sale prices, who can resist). I've been taken in.
Permit me to offer these comments on the use of the x pro with legacy glass. I use my collection of Leica glass and legacy Japanese optics with inexpensive adaptors (Rainbowimaging, really decent stuff from this vendor)
One has to see/evaluate/appreciate the quality of images produced by the x pro 1 using the Leica Summicron 40mm/f2 and/or its twin/clone/gentotype , the Rokkor Leica 40mm/f2. This is a 60mm (35mm format equivalent) and perfectly useable and practical on the x pro (IMHO, of course and please don't think otherwise) for normal shooting.
The numbers work out that the "normal" lenses of the 1960-970's era (50-58mm f1.4 to f2) function as really attractive short telephotos for the x pro (83mm t0 87 mm (effective 35mm format equivalent) when one allows for the difference in sensor dimension. On a good day, a fellow might buy a 50-58mm legacy lens for USD10 (be a sport, go for USD$16 if the seller is intransient, I like the Super Takumars and Rokkors) . These seems attractive, to me at least, for a really fast (f1.4 to f 1.8) high quality short telephoto. I really admire my Super Takumar 50mm/f2 on the Rainbowimaging adaptor.
For wide angles, the SLR legacy lenses don't offer much. Spend your oney on modern optics here.