Does it matter from whom we receive a letter of recommendation?
If all of your choices wrote the same letter, then of course the letter from the distinguished professor would carry more weight than that of f the assistant professor. 

However, often the letters from assistant professors are much more detailed and insightful than the letters from distinguished professors, because they have usually worked with the applicant much more closely. They can therefore offer more detailed insights than can more senior staff. This is not always true, but it is at least mostly the case in my experience.