The more you put in, the more you get out. If you're just mindlessly vibrating a mantra, it doesn't matter what the mantra is; the effect will be severely weakened. Attention is the name of the game. If you empty your mind of all thoughts and just focus on vibrating the mantra, you will feel the energy.
I am writing this from personal experience, as I did this for the longest time. I thought there was some secret sauce, when in reality I wasn't really meditating. I was vibrating the words, doing the visualizations, but also thinking about 100 different things at the same time. You will be stunned at the difference in power if you just focus on doing one thing.