If you’re unsure which to buy, Tumblita or Beelog, I’ll save you $12 to buy them both and just tell you that they’re both bad.
Tumblita lets you have multiple tumblr accounts and post to them. It doesn’t let you read any of the tumblr blogs you follow, though.
Beelog looks and feels like the email client Sparrow, and lets you easily view all your followed tumblr blogs. But it doesn’t let you use multiple accounts.