The question is... how can you compare two candidates?
GRE/GMAT is easy to compare... check! Better GRE/GMAT => higher chances. Sure, cultural/educational background will affect the English based writing and verbal sections, but the fact of the matter is that American universities teach in English so better English skills generally mean a better chance of success.
GPA? It gets fuzzy here as a "good" GPA can vary widely across different schools, different programs within a single school, and even within programs (depending on the courses chosen by the student). Ceteris paribus? Well all else is never equal when it comes to applications. Also, once you are already applying, GPA is largely out of your control.
So much for quantitative comparisons! What are we left with? Undergrad school brand (not under applicant's control at the time of application), statement of purpose, and recommendation letters.
Remember, an admissions committee is not a machine that takes in applications and spits out admissions decisions. They are composed of people, and people generally share a few key psychological traits:
1) They respond to emotional statements very strongly.
2) They trust your recommenders more than they trust you.
3) They like it when care is put into a piece of writing.
Your recommenders likely share a few traits as well:
1) They are busy.
2) If they agreed to write your recommendation, they genuinely want to help you.
3) As a direct corollary to (2), if you make it easy for them to write a good custom tailored (i.e. NOT generic) recommendation, they will be more than happy to do so.
4) Conversely, if you make it difficult for them to write a nice letter, they will not do so as a consequence of (1), even though they genuinely want to help.
The application of these propositions to writing statements of purpose and to obtaining useful recommendation letters is left as an exercise to the reader.