Big diff is that everything that was in Record 1.5 is now in Reason 6. Plus 3 new effects.
Workflow is pretty much the same, but you can choose whether to have individual tracks for each instrument in the master mixer or use sub-mixers to get layered sounds for individual parts/sets of notes.
Some say the SSL emulation gets in the way, others think it is great. I'm happy because I was working in both Record (podcasts/mixing and mastering) and Reason (sound design/composition) at different stages in the production process.
A downside is that legacy files do not have their mixer channels automatically remapped to the SSL and there is currently no scripting to facilitate this either. this is mainly because the mixer types and channels that existed in previous versions of Reason are still there - so auto-remapping does not make any sense in the conversion process.
It would take user intervention to determine which mixer channels were converted and which remained as they were; something you would still do if you were manually converting hence no direct support for conversions is my guess - why automate a process that requires just as much manual input as it would if the user did it directly??????
Overall worth the investment, particularly via the upgrade paths provided.