Committing your mental process to paper helps you see if you're making a mistake along the way. But you shouldn't have to do involved calculations to get to the answer in a Quantitative Comparison question. Just as in Problem Solving, never make a comparison by visual estimation or measurement. Instead, make your comparison based on you knowledge of mathematics, along with non-graphical data provided in the question. Consider all the possibilities when it comes to unknowns.
Comparisons often depend on which sort of number is used.
You may be able to combine numbers or other terms, do some factoring, or restate in equation in a slightly different form. This technique may help to simplify one or both of the expressions, thereby revealing the comparison. You might be able to simplify one or both expressions by multiplying or dividing both by the same term. So think twice before performing either operation on both expressions.