With the downloaded each license includes one absolute admin user that logs in as admin. This user has absolute permissions and access to lists. If you want to have additional individuals login with unique login credentials, permissions, and access to lists you need to purchase a license that includes multiple user slots. There are no limits on the number of subscribers you can send to or how many unique lists of subscribers you can create. Any limitations would come from your hosting environment. The advantages of the downloaded version is that full source code is included allowing you further customization options. You can also own the license without paying any reoccurring fees and use the software as a hosted service for your clients directly on your web site.
With the hosted version the pricing is based on the total number of subscribers on your list but you do not have to pay for additional user slots because you are simply being charged based on your subscribers. The advantages of the hosted version is that you do not need to worry about configuring your mail server, worry about your hosting environment, install the software, handling reputation management, configure any POP accounts, or set up any cron job.
