This will be a short post: I like merb/datamapper better. It’s cleaner and faster.
But, I have to finish a project this week and merb/datamapper is not just stable yet, has less documentation, less plugins, less helpers and less features.
For example, the “has and belongs to many” association in datamapper isn’t fully implemented (look a the tests). There are some nice backend generators for Rails. And if you have to finish a project in too little time, it’s a quick decision.
I know, I could also make/fix this stuff for merb and datamapper myself and I would love to. But there just isn’t enough time! Aaarghhh…