The Online Licensing System with API has an application programming interface that can be used to connect it with your website. For example, if you wish to email a license key to your customer when he purchased a package on your current website you can use the API to make it happen (To do this kind of work it's required to do an additional development from your website side.)
The Online Licensing System without API version consists of all of the features shown in the demonstration and advertisements. So, if you are not planning to do any additional integration we suggest you choose the Online Licensing System without API. Also, it's possible to add the API later to the non API version.