First, when you talk about a .mov file, that's QuickTime file format, and is a simple container, that can be any of the codecs in the QuickTime library. When editing in FCPX, we're specifically talking about ProRes. So it's MXF vs ProRes.
Is MXF better to edit with than ProRes? That's an invalid question. It depends on your hardware. Will your hardware support playback of highly compressed MXF files smooth enough to edit with? If not, Optimize to ProRes.
There is no one format better suited for 4K editing than another. Depends on your hardware and preferences.
On a laptop 4K editing is possible, but not as smooth as on a desktop, because desktops have more powerful GPUs. It's all about the GPU, and the external drive your material and Libraries are on. Don't edit off the system drive, because, especially with 4K material, it will cause a data pipe bottleneck. The OS and apps have to write/read invisible temporary files in order to function smoothly. That takes up some bandwidth. Like the limited bandwidth of your Internet connection, same goes for the connection between your CPU and hard drives. Thus, we put media and Libraries on a very fasts (Thunderbolt or USB 3, either SSD or 7400rpm HDD) external dive for a laptop.
Thing is, MXF is highly compressed. Each frame has to be de-compressed to play it back each time. That takes hardware horsepower, benefited by a powerful GPU. ProRes doesn't have to be de-compressed in that way, so it plays back much more smoothly.
Another option is instead of Optimizing the 4K material, make Proxy media when importing. Then switch to Proxy when editing. Switch back to Original when exporting. This switch is found in the Viewer menu (top right of Viewer pane).
At finalcutprox.guru on the Support Links page are a vast catalog of user manuals (best place to start) and workflow documents (very helpful) that can get you started in wrapping your head around all of this. It's not "simple" and there's no real way to make it simple. But this is all very easy to learn, don't be intimidated. Like anything else, it seems complex and incomprehensible when the novice takes their first look. But go step by step, and as you learn, you'll see really it's not that difficult or complex.
In a nutshell, my advice is, don't optimize. When you import your 4K footage, in the Import window, check the option to create Proxy media, and switch to that during editing.
https://help.apple.com/finalcutpro/mac/10.4.1/#/verb8e5f6fd
https://help.apple.com/finalcutpro/mac/10.4.1/#/ver2fd7a8b94
