What is the proper Vajra Guru mantra?

I see "Om ah hung vajra guru pema siddhi hung" but also "Om ah hum vajra guru padma siddhi hum". I know the first is Sanskrit mixed with Tibetan and the latter is pure Sanskrit. I also am aware that it is about how the Tibetans pronounce Sanskrit. Is one preferred or more authentic? Is it up to the individual? Any elucidation on this matter is greatly appreciated. Thank you.