Wheel size depends on what you want to do/type of riding. The 29 inch wheels will roll faster and roll over objects better. The 27.5 inch wheels will be more nimble and turn tighter. If you are going to be racing you’re probably going to want 29. If you’re just looking to go out and have fun you may want the 27.5 wheels. For example, when I bought my Rockhopper I knew I wanted a 29er because I would also be riding paved/gravel trails with my wife and I wanted something that rolled faster. After riding a few 27.5 bikes I found them more fun on the trails so when I bought a new bike I went with 27.5. I kept the Rockhopper so I can still use it to ride with my wife and it serves as a backup bike as well.
Another thing to consider is what frame size you need for the x-caliber. How tall are you? The x-caliber has 27.5 wheels if you get the small frame and 29 if you get medium or larger. This is something that Trek does that annoys me. Let me choose the wheel size I want.