As an avid coin collector for years, people have often asked me what is the difference between grading companies and why is there such a huge difference between prices for the same coin graded by different grading companies.  The answer to this question is simply:  Perceived Consistency and Standards.  Your higher profile grading companies are perceived to have a greater consistency in grading and their "grade" standards (MS66 at one grading company versus another grading company) are typically much higher (especially in the Uncirculated Grades).  As a result of the consistency and stricter grading standards, these higher profile grading companies command higher premiums for the coins they grade.  For example, if their standards are high and consistency within grading high, the numbers of a particular coin that obtains a certain grade (ie. PR70 or MS65) are much lower for certain grading companies.  This creates scarcity of the coins reaching this level or grade and combine that with their consistency in grading creates a higher cost to own.  Simple law of economics - Supply and Demand.  For example, look at the price differential in PCGS graded PR70 Deep Cameo Lincoln Cents or Washington Quarter or Kennedy Half Dollars versus NGC. The difference in price is simply because the population of Lincoln Cents, Washington Quarters and Kennedy Half Dollars receiving a PR70 DCAM designation from PCGS is much lower than NGC.  This scarcity in population versus the demand from prospective buyers creates higher values.  Thus, if you collect coins for an investment, you are much better off buying from these high profile companies (PCGS, NGC, ANACS) where the standards and the consistency of grading are high.  If you just like to collect with no reasons for a return on your dollar, the lower profile companies are probably better for you.  Hope this helps and Happy Collecting.