What type of surface/terrain do you intend to ride on? Mountain bikes have front suspension (adds weight) and fat tyres which are harder to ride on roads but better on rough tracks. Aluminium frames lighter than steel.
Had many years of riding mountain bikes (carried on back rack) but found that we ride 99% of the time on roads and cycleways and so put slick and narrower tyres on. A couple of years ago i changed to a hybrid bike which took less effort to ride on the road but with having reasonable tyre width was still ok to ride on tracks. No front suspension so saving on weight (watch out for cheaper aluminium bikes as they sometimes have steel (heavy) front forks -check with magnet). Wouldn’t go back to mountain bike now. The more you pay, the better the ride. Decent second hand bike like a Specialized Sirrus (men’s) and Altus (women’s) can be picked up for about £200. At home I tend to ride a road bike but away I find the hybrid a good compromise.