I would think thoughts are more powerful because the conversion of thoughts into words can never be total - there can never be 100% fidelity because of the limitation of words and the fact that thoughts encapsulate far more in terms of subtlety, tone, texture and emotional and spiritual energies than the words finally convey. A skilled speaker may approach 100 - be close to 100 - but but never be 100%.
The power of thoughts is not experienced by us because we measure power in terms of the impact on others. It is very difficult to fathom or visualise the impact that our thoughts will have, whereas the impact of our words is immediately felt and experienced in tangible terms.
It's like comparing the impact of a tree on its surroundings and the impact of the seed from which it grew.
So I would say that the thought from which the words grew is far more powerful.