I've been shooting w/ an X100 as my carry everywhere this past year (pretty invested long time Canon shooter on the DSLR side). I recently was making the same decision and it was a relatively easy call for me.
The X-Pro1 has two main things really going for it: 1) the Hybrid OVF and 2) the X-Trans sensor - APS-C sized, no AA filter, that new CFA. Also while it seems like they won't be fixing the X100 (grumble grumble whatever), it looks like they've at least paid attention and fixed most of the the issues w/ the XP1: fast startup, responsive when writing, q-menu, extra fn button. We've yet to see if the MF is much improved (can't be worse), or how the AF performs. Unfortunately, w/ the focal plane shutter, you lose the X100's quietness and fast flash sync, and while the dial locks are great, they haven't added an ISO dial (although at least AutoISO is in the ISO menu now).
I've been happy w/ the X100 and not super hot to trot to buy a new system, but the OM-D was pretty compelling, so I pre-ordered. My list/thinking...
- Weather Sealing: this was a biggie for me. I do a lot of traveling for work and leisure, and I definitely needed something more rugged than the X100 (which fogs over at the slightest hint of moisture). The E-level weather sealing is fantastic. Hopefully there'll be a weather sealed pancake prime lens soon...
- Size: While the XPro1 is bulkier, an OM-D w/ a pancake prime (like the Panasonic 20mm or Oly 17mm) is pretty much the exact same size as the X100. This is great IMO. The additional grip + vertical grip is also a nice option when looking for more to hold onto.
- EVF: I can live w/ the Oly EVF. While I love the OVF on the X100, it's one of the great pleasures of shooting w/ it IMO, so it's understandable that this is probably a dealbreaker for some. For me, the EVF on the OM-D should be a compromise I can live with (also the frame rate on the VF2 is much less laggy than the X100's EVF).
- Sensor: while it's smaller, and has lower DR (nothing in the samples suggest any of the silky smooth goodness of the X100), using the latest Panasonic m43 sensor, high ISO shooting actually looks pretty decent - in real life probably better than my X100, which tends to band above ISO 3200. Also, that 5-axis IBIS looks dead sexy and should be fantastic for long shutter handhelds (smaller sensor and more DOF will help too). Obviously the trade-off is that DOF isolation is that much harder, but overall IQ looks good enough.
- AF: The E-M5 is going to have best of breed AF, including shooting 4fps w/ continuous (3D matrix) AF. There's face detection (would save so much misfocusing when doing social shooting w/ the X100), and touch-point AF as well. Also, the fly-by-wire focusing is actually known to work (shocking, I know).
- Handling: w/ two dials and two fn buttons (also a configurable movie record button) it should be perfectly fine. The grips can apparently have different functions assigned, which is nice...
- Lens selection: probably most importantly, m43 is head and shoulders where either the X-Mount or E-Mount will be anytime soon. There's a big selection of decent to great primes that are pretty compact (the Panny 20mm, Zuiko 12mm, Zuiko 45mm are at the top of my list right now), as well as some OK zooms (not really my thing). While the promise of M to X adapter mount is nice, MF w/o peaking seems pointless for all but the most leisurely shooting. (Oly also has some philosophical disagreements w/ peaking, much to its detriment, but w/ a much better native lens selection and an AF-enabled 4/3 adapter, it's not a dealbreaker).
Lastly, a couple other reservations about the XPro-1. There's the issue of cost - the XPro-1 is going to clock in at twice the price w/ a lens (and the 18mm isn't looking so hot). And for me, the bigger thing, the lackluster updates on the X100 would make me wait until the reviews rolled in for the XPro-1. While Fuji has made many changes in response to X100 complaints
for the XPro-1 , over the course of 4 or 5 firmware releases
for the X100 , there haven't been substantive fixes to most of the X100's issues, from simple things like the ISO menu (the dial still works reverse in choosing ISO from EVF vs OVF, AutoISO is still buried multiple menus away) or the RAW/fn2 button, to much serious issues like the useless (and light dependent!) MF, AF speed, aperture dance, or AEL/AFL (AFL-S also AEL regardless of your AFL/AEL setting). Based on the past year w/ the X100, I think it'd be overly optimistic to think that any issues w/ the XPro-1 would be fixed/updated...