This depends on how much of each type of riding you will be doing. Well, it depends on a lot of things, that only you will be able to balance out in your mind, eventually.
I had a Fuji hardtail some years back, and a Fisher MTB (fully suspended), that I considered your option #1 for. The problem is that unless you can get the exact same wheel/hub combo, and have the widths and spacing exactly alike between your two wheelsets, then there is inevitable adjustment to the derailleur that needs to be done every time you switch the rear wheel out. If you have time, and the inclination to do this, then maybe that's an option for YOU. It wasn't for me.
I would say that if you will be doing 30-40% of your riding on the road, and if those rides will be of any substantial distance, get the 2nd bike. All of the major bike manufacturers make decent entry-level road bikes. The Trek 1000 and the Specialized Sirrus come to mind.
Used road bike for less than $300, I guess if you can find it, that is your best option. Make sure it is fit and is at least as fun to ride as your mtb is. If you are riding an old beater road bike that doesn't fit, then you have lost instead of gaining.
Unless your tires are currently just extremely aggressive, i would suggest just using what you have or taking the time to swap tires. Otherwise save up more money and get a better road bike.
I switched the knobbies on my Stumpjumper for Hutchinson Pythons for a nice improvement on pavement riding. $44 at pricepoint.com for the set. It won't get you road bike performance but it's a quick fix step in the right direction.