I sold my D300 & lenses, including the 18-200, 18 months ago to buy an XT1 from one of the first batches available here in the Middle East. I really liked the D300 & 18-200 for travel & while it is not a super sharp lens by any means, it made pix I was quite happy with & that won several local competitions, so the 12MP & average sharpens of the lens were no technical barrier.
It's a tough place for cameras here, with the temperature extremes and ever present dust in the air, so changing lenses is not something I liked to do very often, so the 18-200 stayed on most of the time. In the end, it was the weight & size that became a deciding factor in favour of the Fuji. Small planes & big camera bags do not go together. Pro friends, such as Zack Arias, Dave Hobby & Bobby Lane all switched to Fuji or at least added Fuji's to their gear list to try the system out & see if they could ditch their full frames & many bags of gear when on travel assignments. I'm not within long distance driving of their talent but I thought if they could make a living with the Fuji's, then I could surely make some pix I'd be happy with. So, I swapped & on the whole, have no regrets. Aside from the light leak when the accessory door was open which affected some of the early XT1's, I've had no issues at all. The lenses are noticeably sharper than the 18-200 but I have to swap between the 18-55 & the 55-200 to get the same range. While these aren't rated as weather resistant, they have coped with the tough conditions here & also been out in bad weather on the Norwegian Atlantic coast & around London on stormy days without a hitch (of course, everyone's experience could vary & Fuji do not rate them as weather resistant).
The biggest issue is getting used to the different feel in the hand & placement of controls. The tech specs from camera to camera are pretty much the same & just about every modern camera will make a good photo in experienced hands. For me, it comes down to that tactile experience & whether you can adapt to the different haptics in a reasonable time. The XT10 is a smaller camera than the XT1 & if you are a person with medium to large hands, this might be a frustrating experience that no amount of usage will overcome. My best advice would be to hire one if you live somewhere where you have access to see if it is a good physical match. Other things to consider are whether you are left or right eye dominant & the placement of the viewfinder & the AF-L & AE-L buttons. If you like to back button focus without taking the camera from your eye & you are left eye dominant, then your nose may get in the way of a smooth access to these buttons. These sorts of things are the more practical considerations that you would need to live with. While it is true that we really buy into a system & the lenses are the backbone of that system while the cameras will come & go, we still tend to keep cameras for several years & several years of frustration will bug you each time you take it out of your camera bag. I really love my XT1 & have had amazing support from Fujifilm Middle East, but I think you should try before you buy if this is at all possible.
