Jekyll requires you to generate your site. Dropplets generates pages on each request.
Jekyll runs on your machine, and is written in Ruby. Dropplets runs on your server, and is written in PHP.
Jekyll's posts require very minimal metadata, and use a key/value pairing system with YAML. Dropplets' posts require a ton of metadata, and the same metadata for every post, and in a specific order, and you can't add arbitrary metadata.
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I don't think Dropplets has categories, but I may be mistaken.
Also, how about support for category pages? I mean, show a page full of posts from one particular category only? Possible??