IME, stock can be difficult, especially if you have a collection that doesn't really stand out. Cats, dogs, flowers, landscapes are all categories that are very saturated. It is also a huge investment in time initially for very little return. If you can find a niche, you'll start seeing returns eventually. When (and if) your library starts selling regularly, the time investment can become much less if you desire. I shoot stock during the slow times and just ride the wave when I'm busy. I don't make a huge sum of money with stock but I'm happy to see those $250+ checks from Alamy every so often....even when I haven't uploaded or shot a stock image for months. Marketing and selling your own images is also a good route IMO but I personally prefer to put my marketing energy towards family oriented portraits.
In my experience, it does take a large library of thousands of good quality images to start seeing those fairly regular sales....and that takes time to put together. Don't expect much when you're library only contains 100 images.