LISTSERV and CREN ListProc are commercial products, licensed by the year (more or less--each has several licensing plans and you should contact their vendors for details). In exchange for your money you get a product that's stable, documented, and pretty simple to set up, with paid support people to help you out if you get into trouble. The other MLM's are free, with varying levels of support from their users and maintainers. Each package has a mailing list for users, on which you can ask questions like "digesting doesn't work, what do I do?" Many people are satisfied with this sort of support; you may or may not be. Talk to current users of each package you're considering to get a feel for whether you're comfortable with its combination of administrative burden and software cost.