Opinions based on spending less than an hour with each bundle:
Orchestra gives you more framework to build on top of. It has its own login system and generates a few, mostly empty pages, and then creating views, models, and more for rest of your administration area is up to you. There is lots of documentation available which requires the Bundocs bundle to view. It is definitely easier to install this bundle on a fresh install of Laravel. For example, its user’s table may exist with the one in your database already.
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Administrator lets you perform CRUD operations on the rows in your database. It uses your app’s eloquent models and additional configuration files. The configuration files make it quite powerful since it lets you set up filtering, specify different form elements for editing different table columns, create new getters, and manage some relationships between tables. Documentation is here.