Earlier quoted context omitted.
Good question. WakaTi.me knows what project your working under while rescuetime.com only knows that your inside your text editor.
That makes sense. RescueTime gives me information about how much time I spent in each file and is showing me the project as part of that info, but it doesn't rollup time spent in a project. Still seems of limited use since it wouldn't show me time spent debugging in a browser, working in the database, etc. From a productivity standpoint RescueTime lets me dictate that http://myproject.dev is "debugging" time, Sublime…
The key word search method is how people usually accomplish the idea of project reporting-- but is admittedly not perfect, like you could get if we actually could consume that meta data.
However, the "document" level details are only exposed to you in the premium version-- in the free version you just get an open document count, so that is a significant difference.
I've always thought the long term path for an enriched time measurement experience would involve plugins like this one. Great to see someone working on it.
Disclosure: I'm a RescueTime employee.