To sum up, it all depends on your painting style, whether you like large areas of watercolor washes or more controlled paint application. If you are just starting out and trying to experiment with different methods of paint application, I recommend getting loose paper so you can test the different paper types before committing to a large quantity of paper.
The most cost-effective option, in the long run, is a roll of paper, which is great once you know which type of paper you work best with. There are times, however, when you want to paint away from the comfort of your home or studio and pads, blocks and boards are the best traveling companions.