Sure, I can see you're keen on getting a bike that suits your needs. However, I'd like to point out that versatility and durability are subjective terms. Both gravel and mountain bikes have their strengths and weaknesses.
Frame design is crucial when it comes to bike selection. Gravel bikes often have a more relaxed geometry than mountain bikes, which can make them more comfortable for long rides on rough surfaces. Mountain bikes, on the other hand, have a more aggressive geometry, making them better suited for technical trails and downhill riding.
Suspension is another critical factor. Gravel bikes typically have little to no suspension, while mountain bikes usually have front or full suspension. If you're planning on riding technical trails, a mountain bike is the way to go. However, if you're sticking to smoother roads and gravel paths, a gravel bike with wider tires can provide enough cushioning.
Speaking of tires, gravel bikes usually have wider tires than road bikes, ranging from 32mm to 50mm. Mountain bikes have even wider tires, ranging from 2.1" to 3" or more. The wider the tire, the more grip and cushioning it provides, which is crucial for off-road riding.
As for typical use cases, gravel bikes are great for long rides on gravel roads, dirt paths, and even smooth pavement. They're also suitable for bikepacking and light touring. Mountain bikes are designed for off-road riding, including downhill and technical trails.
Ultimately, the decision comes down to your preferences and riding style. Don't just take my word for it - do your research, test ride some bikes, and make an informed decision.