Should I buy a gravel bike or a down country mountain bike?

Although there is reasonable overlap in what you can use gravel and downcountry bikes for, the answer her largely depends on what you plan to do the most of and if trail riding will be important for you. Gravel bikes are great for covering offroad miles more efficiently and long days in the saddle both on road or offroad. However, the lack of any meaningful suspension means that or but a small amount of time off paths and more established trails will be hard work. Downcountry bikes will enable you to go on a wider range of terrain, inlcuding the ability to do trail riding (be very careful before trying to navigate downhill trails on a gravel bike!). However, downcountry bikes are usually heavier but the full suspension and geometry makes them less efficient compared to a gravel bike.