A lot has already been said, and you seem to had finally found what you needed. But just in case someone has the same question... I have both lenses, and I wouldn't say one is better than the other, as they both serve a different purpose.
The 55-200 is a great zoom lens. The IQ is very good, judging by my recent tests. If you just need a light telephoto lens (compared to other options) with a good image quality and don't need WR, go for this lens. When I go hiking in Bavaria, I usually take the 14mm and the 55-200 lens, and I'm a happy man!
The 18-135 is another story. It reminds me a bit of the Nikon 18-200, which was a very versatile lens. As someone said above, it's a Swiss army knife! The WR, the OIS, the versatile focal range... it's a great companion for travel. I think it's a good lens, globally speaking. It's definitely not bad, and it's not the best either, but it works well. I have made some pretty nice pictures with it.
I would definitely bring the 18-135 to a "once in a lifetime trip" -- but it depends where! If it involves a lot of nature, like in a demanding environment, then yes. I don't want to change lens in the middle of a desert or jungle, and I might need to do alternate landscape and wildlife shots. But if I'm going to a nice city, like New York, where I would occasionally need some reach, then I would take the 55-200 instead. And probably use my X100T (+WCL) for most of the shots (mostly street) and the 14mm for the landscape shots.
Also, I believe that what matters most in a once in a lifetime trip is not image quality, but the ability to catch moments. And the 18-135 gives you far more versatility and flexibility in that regards, due to the focal range and the WR. Sure, you might loose a bit of image quality down to the pixel level, but who cares? You were here, you made a nice picture of a beautiful place... it's all that matters
I thought this was about Steve loosing it? Well that actually happend a very long time ago already according to http://huffparanormal.com/ . Stopped reading his stuff after I discovered this..
And yes, the A9 is an impressive camera, but does that mean it invalidates the X-T2? I think not..
