Unless you truly plan to do some trail riding, I definitely think your second idea is better. You already say you feel comfortable on it, and it meets just about all of your needs. So instead of buy another bike for the 20% or so of the time you feel you need more, why not wait till the tires are dead and switch to something a bit meatier. The things you described don't really require a mountain bike and it would kind of suck to make a large purchase you don't need, or may not even like.