Fuji vs Fuji did an excellent write up of this as well. The 18-135mm is shorter and lighter than the 55-200, even though it is a little fatter. To me, it feels perfectly balanced on an XT-2.
So you have a little bit of objective advice, here's some shots with both lenses. I wasn't being exactly careful and the shots with the XF 55-200mm ended up being at 128mm. This test is far from perfect but it'll help you get the idea. They were all taken on an XT-2 and had no filters.
As you can see, when racked out, the 18-135mm is incredibly soft at larger aperatures, especially when compared to the 55-200. It doesn't really start getting its act together until f/10-ish. Hopefully this can help the OP or someone else stuck in this dilemma. If low light is causing a problem for you with the 18-135 its probably going to do the same with the 55-200. It really only has at best a one stop advantage in the overlapping zoom range. Telephoto zooms of this size aren't exactly known for their low light performance.
If the sharpest photo is of the utmost importance, then get the 55-200. If ultimate versatility and WR are important, go with the 18-135. Sometimes the subject matter is so compelling that noone will notice your photos at 135mm are kind of soft. It's a great thing to have such range in one lens.
