Developed by David Belfast and Lisa Clarke, TwittDiff is a Twitter follower differencing tool that works similarly to Qwitter. What this simple but useful service does is that it informs you via e-mail or RSS whenever a particular user has unfollowed you. Although the features both applications offer are the same, but according to the folks there, TwittDiff is much more reliable as compared to Qwitter. This is because it runs on Google AppEngine and gains secure access to Twitter through your account directly, without the need to provide your Twitter credentials. As a result, a high follower list will not even exhaust the API quota.
On the other hand, Qwitter is using anonymous access rather than secure access. Therefore, it could make up to 150 checks per hour. This might cause some hassles, if any users had unfollows this would be even less, since it must also fetch profile information. Besides that, users may worry about their private information such as password will be given away and the developers can make use of them to log in and gain access to their accounts.