Unlike Nintendo, Sega sold their MD/Genesis-Games in many different cases. With the initial release of the Mega Drive the used the grid-style covers. With the re-released of the MDII they switched to the new blue bubble/steele(??) labels to reflect the now more modern design of the md.
Some blockbuster-games like the sonic titles were first released in the old format and in later production-runs in the new ones. The actual case were more or less the same, but the inner construction differs from game to game (like style of the catrigde-holder, clips for instructions etc). The most anoying thing IMHO was that some cases had f*** holder-clips on the top used to store/display them in shops.
Some major publishers (like EA or Hudsun) had their own case and catridge-design. EA-Cases were quite fragile and often get out of shape. At the end of the MD some games (e.g. Sonic & Knuckles) were even sold with those crappy paper-cases they later on used for the PAL-32x-Games.
If you want a uniform appearence, its really hard to collect Sega

Good to know there are now alternatives here!