With 5.5, it was usually a good idea NOT to update - as they usually had more bugs than in the version you were using. There were some EXCELLENT point releases though:
5.5.9.17 - last one before a new feature in 5.5.10, and VERY stable indeed
5.5.10.42 - a pretty mature release of 5.5.10 - also stable
5.5.10.54 / 5.5.10.56 - fixed the bug introduced in 5.5.10.50 (same bug as in an early V6, cannot burn close to maximum or overburn).
Nero V6 - they released a BETA, that's about the only way to describe it - it wasn't adequately tested, and it had WAY too many bugs.
With Nero 6, there are STILL important fixes coming down with every update - with this, Nero sunk to the same level as Roxio - buggy releases that are only right in time to be superceded by the next major version.
Nero Burning ROM is the main program - not sure how the OEM version is defined in V6.
In V5.5 - you had:
1. Full retail / full registered (similar)
2. Full OEM (mild feature clipping)
3. Full Bundle (as OEM, but drive-tied)
4. Express (only able to use the "easy" Express interface)
In V6, there are variants that I'm not fully up on.