They each offer some advantages and disadvantages. However, their essentials are pretty similar once you dig deep. They are both blogging platforms. In short, Twitter is technically a micro blog, and Tumblr is one of the best, if not the best photo blogging platform out there. Blogging is a community activity. It is an act of putting your words out into the world and telling others to come check them out –sharing your thoughts, opinions, and vision with the world. Without that sense of community the blog is not living up to its purpose.
Both Twitter and Tumblr are very similar in a lot of ways.