Well, depends. What kind of content?
Research heavy whitepapers, case studies, or simple SEO content?
However, as many said by word tends to be the better option unless you have worked with the writer ,who you will be paying hourly, before. Most will just take their sweet time vs. by the word they will do that as fast as possible(thus the need to set expectations based on quality you want). Just do a few samples by the hour and you can get an idea of how much you pay per word and compare it to premium services.
When your car had an issue, don't you prefer flat pricing to fix the issue vs. posting a mechanic hourly to find/fix the issue?
