Free ebooks often attract more reader activity because there is no purchase cost, but that does not necessarily mean a free promotion is better for every author.
A free promotion may be ideal when the goal is maximizing downloads, introducing a new author, or bringing readers into a series.
A 99-cent promotion generates paid sales and may attract readers who are more willing to spend money on the book from the beginning.
The better strategy depends on what you are trying to accomplish. Download volume, revenue, series read-through, rankings, and long-term reader acquisition are different goals and may call for different pricing strategies.