Hi Matthias, and thank you for your questions.
At this point, there aren't drastic product differences between the Wolfram Development Platform (WDP) and Wolfram|One. The only difference is licensing - subscription options, resource limits, etc. In the next few months, we will offer a perpetual license option for Wolfram|One, with service renewals for cloud access. (That is not planned for WDP)
Wolfram|One is not a product for Mathematica users. It is set to become the base product for everything besides Mathematica/Mathematica Online. We will build upon Wolfram|One to make it more flexible and make it the single best entry point into the Wolfram Language.
Wolfram Development Platform is focused on WL developers who want to create and deploy product software systems/applications. Wolfram Programming Lab (WPL) is focused on education and exploration. If you believe you don't match either of those descriptions, then Wolfram|One is probably your best best.
Thank you for your interest.
Clayton Voyles -
Wolfram Research