Trail running and road running share the same basic motion but demand different things from the body and reward different goals. Choosing between them - or deciding to do both - comes down to what you're actually training for and what kind of running you enjoy enough to keep doing.
Road running's repetitive, flat-surface impact tends to stress the same joints the same way every stride, which is part of why overuse injuries are common. Trail running's uneven terrain varies the impact and engages stabilizing muscles more, but raises the risk of ankle rolls and falls instead.
A trail mile and a road mile at the "same effort" can take very different amounts of time once elevation and terrain are factored in. Runners who switch from roads to trails often need to let go of pace as the main measure of a good run, and focus on effort or time instead.
Trail shoes with more aggressive tread and rock protection matter more on technical terrain than they do on pavement, where a standard road shoe performs better and lasts longer. Runners who do both regularly usually end up owning one pair for each.