Migrate the folios individually, take the time to add keywords and metadata and use the filter functionality to reduce the article list down to manageable chunks.
We are working on some UI additions to help in the organizational areas, they include adding the article name to the list of columns in the UI, letting the user choose what columns they want to see, building a visual representation of your content hierarchy but all will take time to build, test and release.
Right now I advise against migrating all your content all at once specifically if you are just kicking the tires of DPS 2015. Migrate a few pieces, invest in determining your data hierarchy, spend some time looking at keywords and the various metadata fields, Play with preview and layouts with a sample subset before migrating significant amounts of content. We aren't going to suddenly shut down the old solution or remove the migration function.
When you are ready to shift from the old to the new, you'll have a better idea how much to migrate at once, how to tag the migrated content and how to assemble it.
