Cloudera (the company behind CDH) and Hortonworks (the company behind HDP) have merged. They now are called Cloudera.
After the merger a new distribution was released, called the Cloudera Data Platform, or CDP in short.
Though both older platforms will still exist for a short while, all new users should go for CDP. This is the platform that is seeing all development focus.
Note that the situation may be more nuanced if your company is already a heavy user of either HDP/CDH but even in those cases the formal recommendation is still to go to CDP as soon as possible.
Full disclosure: I am an employee of Cloudera (formerly Hortonworks), the company behind CDP as well as HDP and CDH.