As far as the true differences, there arent any. 6 digit just means that person registered their CD key earlier than an 8 digit. From what I have seen that is. For the most part online all I see nowadays are 7+ digit accounts. The occasional 6 digit and the random less than 6 digit that still play online.
Neither is better than the other.
But there is one hitch to the 7+ digit account. If you join a server with players of 6 digits or less, chances are they will type status in console and watch whichever player is pwning. In this case I will say you are destroying with 6+ kills a round 4 headshots. You are more likely to be spectated by admins and other players due to them swearing you cheat.