Rain Barrels are a great introduction to the world of Rainwater Harvesting. However, they are only good for small gardens or patios. If you are just suplimenting the occasional dry week great, but if you want be ready for the next drought or long dry period get a tank. Look at your local rainfall history. Start planning 3-4 weeks of reserve and leave room for another tank down the line.
Also take a look at my rain barrel page at the top to find out good tips on building your own or what to look for when you buy one. Remember, have at least one 1 1/2″ overflow directed away from the house and ending in an area that will not erode. Secondly, make sure your barrel is opaque so not to allow sunlight to cause an algae bloom.