It depends on you what you are looking to learn.
For Example, If you are interested to learn how Cloud components like networking ,storage ,compute ,caching etc. works and you want to go into nut and bolts of each component and play with it. Then you should go for OpenStack ,install it on your server and start exploring it.
On the other hand your aim is to build a cloud application and you want to deploy the application on AWS. Then start learning AWS.
Hope this will clear your doubt.