You already have two bikes, so the absolute worst answer is to buy another bike :-) While you like the cruiser, you should ride it whenever it is appropriate. You aren't likely to find it accommodating to off-road riding. So I don't think the answer to pick one and ride it is going to work. You *can* get thinner tires without knobbies for the mountain bike and it will be an improvement on the road. But why give up the cruiser if you like it? You could probably make the mountain bike a bit more comfortable if you get a shorter stem and get the stem angle so that the bar is more upright. But it's probably not going to feel like the cruiser. So ride them both! I think the correct answer is that you'll do more riding if you have a bike you enjoy riding. Riding the bike instead of the couch is going to provide the best fitness.