Let me start by saying that if you are buying coins on the internet buy coins graded by PCGS or NGC. The other companies are variable in their grading with the exception of maybe IGC and ANACS. Trust the grades of none of the other companies you see.
The difference between PCGS and NGC is the strict grading process at PCGS and the resulting grades are worth more. I have tested this on a few occassions with the result being that PCGS standards are much more difficult to obtain. For example I bought a 1971-s pr69dc graded by NGC and removed it from it's case and sent it in to PCGS and received a grade of pr68c, then I did the same thing back and forth two more times. Both companies graded the coin EXACTLY the same each time. I have only received the higher grade from PCGS a couple of times. Bottom line, a PCGS graded coin will always carry more value for the given grade, so it is almost always a better investment.
DO NOT pay a premium price for ANY "certified" coin that is not PCGS, NGC, IGC, or ANACS. There are a number of sellers that list a lot of coins, (anywhere from a few to many), that have the PCGS price listing for a total value. These coins are usually certified by substandard grading companies and are by most accounts nowhere near the listed value. TREAT THESE COINS AS LOOSE UNCERTIFIED COINS!!!! These can be very substandard and because they are encapsulated it might lead unsuspecting buyers to believe they are getting something that has a certified grade, when that is not the case. The price difference can be shocking when you receive the actual coin and get a second opinion, usually leading to a grade loss of 3 or 6 points.
Keep in mind, if it seems too good to be true, it probably is.
Please vote if this has helped!