I used to like Retrospect
very much. It had been a Mac only application for over a decade and was one of the best pieces of software anywhere with the best company support anywhere. Then Dantz was bought by EMC, and the long decline began as the corporate weenies had it converted to Windows and tried to wring the maximum profit from it to the exclusion of its core values.
Anyway, that's kind of beside the point here.
I wrote a post on
why I like Amanda that is getting a bit old now, but is still appropriate. There has been a lot of hard work on development since then, with major improvements in access through APIs and added functionality. The 2.6.x versions show the API additions, and the 3.1 version begins to really take advantage of those. The development team is really responsive, and Amanda is one of a relatively small portion of open source projects that received security certification from the Department of Homeland Security as a result of fixing all items flagged by automated source code scanning.
If you have any specific questions about functionality or feature sets, be sure to ask.