It's a compromise either way. Bike seat isn't a car seat or sofa. Your weight is distributed between seatbones, hands and feet.
Softer seat - sinking deeper, discomfort on longer riders. Wider seat - interfering with downstroke. Upright position - more pressure on the butt. Lean forward - more stress on hands and possible back problems.
Some of this will not be applicable to your MTB. Ex., you won't get really upright position because handlebar is "wrong". Try replacing the seat. Cloud 9 is popular (I think they have 2 models now, newer is slightly better).