Every dash cam buying guide eventually asks this question, and the honest answer is: it depends on what you're actually trying to capture evidence of.
What a single-channel (front-only) camera covers
Everything happening in front of your car — the most common location for the kind of incident a dash cam is meant to document, like another driver merging into you or running a light. Simpler install, one cable to route, lower cost.
What a dual (front + rear) setup adds
Coverage for rear-end collisions and reversing incidents, plus a view of your own back windshield — useful if you drive with passengers often, or in stop-and-go traffic where rear-end contact is more common than head-on incidents.
The real tradeoff: installation
A front-only camera is a same-day install for most people. Adding a rear camera means running a second cable the full length of the car's headliner and down a pillar — not difficult, but a meaningfully longer job, and one more thing that can go wrong if a cable gets pinched in a door seam.
Storage and file size
Dual cameras record two video streams simultaneously, which fills a memory card roughly twice as fast as a single-channel setup at the same resolution. Budget for a larger or higher-endurance card than you would for a front-only camera.
A reasonable way to decide
If most of your driving is highway commuting, front-only covers the incidents you're statistically most likely to encounter. If you drive mostly in city stop-and-go traffic, or you want documentation for both directions, the extra install effort of a dual setup is usually worth it.
Whichever you choose, format the memory card in-camera before your first drive — a card formatted on a computer doesn't always work correctly with the camera's continuous-loop recording.