Keep in mind that monitors and EVFs add another level of complexity with cables, batteries, mounts, and settings. The loupes like the Zacuto Z-Finder are dead simple. They just snap on the back and you're there.
A big part of your decision should be based on your shooting style. Are you in the studio on sticks? A monitor will be fine. Are you shooting handheld outside? Loupe will be light, block the sunlight, and provide a point of contact to steady your camera. I use both, depending on the situation I'm in but most of the time I'm using the Z-Finder instead of a monitor.