I've read in multiple places that it is important that your recommenders be known to the admission committee. If I were to have a letter of recommendation from a professor who is quite new (Assistant Prof), would their recommendation be discounted due to their lack of reputation or stature in the field?
As you said: Someone who is well-known is a good choice. If the new professor already has a good reputation in her field, it will most probably be a good decision to ask her. It's better to have a letter of recommendation from someone who knows you and who writes somethong personal, than a letter from someone just looking at your marks.
An unestablished professor that knows you well, and writes a fantastic letter, is far better as a letter writer than a famous professor that will just write a generic "he was 3rd out of 38 students in my class" kind of letter.
There are two types of letters that will really help you in general. (1) A great letter from a professor who knows you well, regardless of their stature. This is likely someone you did research with, and (2) A famous professor that will write you a decent letter, saying you did well in his her/class and you show real potential, ideally you should have done a class project for them, so they can say something beyond basic things like class rank. Letters of type (1) are the most important (if the professor is famous that is a bonus). But you need 3 letters usually, so you are not likely going to have 3 professors that know you well (if you do you are in great shape). Type (2) is another good option for filling out 3 letters. If all your letters are of type (1) and (2) that is a really good sign. Most people will have at least one lackluster letter from an unestablished professor.
The next best bet is an unestablished professor that you did a good class project for or knows you well from a class. Generic letters really do not help your cause.