eleonorang I think a per-word rate is more transparent and easier to explain to a client. They see the reasoning behind it and it makes it easier for them to understand what they'r paying for.
Hourly rates (and even per project rates) can work too depending on the client. Personally I prefer charging per word rates when I have perfectly editable text.
And if I have to deal with non-editable PDFs or something like that I'd probably say that: this project would cost X dollars, flat fee.
Of course you could also convert the flat fee into an hourly fee so that it would be much easier for the client to understand how much work is involved. But you have to be very careful and skillful at giving an hourly rate. You need to be 100% sure that this is exactly how much time you'll spent on a project.
To summarize I think we are free to use different calculation methods based on the client and their needs and understanding of our field.
I'd say use words for agencies and colleagues and hours/flat fees for direct clients.
