Akismet is a spam API service. So there is an apples to apples comparison. And theirs is easy to setup.
SpamCheetah offers both cloud based and on premises install. And SpamCheetah does not do API based spam control(yet). And our user base is small, Akismet has several thousands of customers worldwide and is well known.
We invite you to give our software a try and experience the difference and tell us stories which we can publish on this page.