For me it's worth it having for Multi-processor support and IBL (and Intel native on the Mac). Right now I'm doing my scene creation in 5.5C which is much more stable and then using 6.1 for rendering and IBL. So far this has worked pretty well for me on both the Mac and PC. I solved a lot of my PC 6.1 problems by turning off the Screen and Energy Saver stuff, which interfered with rendering. While this didn't cause a crash it did suspend the rendering process till I woke the PC back up.