Jon has the right of it here. The changes between 5.6 and 6 are nugatory—really nothing at all, if memory serves, save for a switch to a hard-to-read layout and perhaps some expansion to the appendices. Chaosium has traditionally taken an "if it ain't broke don't fix it" policy, and you really don't need 6th Edition if you already have 5.5 or 5.6.
The change from 5.0 to 5.5 was greater, involving (again going by memory here) changes to skills (mostly base numbers 0% versus 1%) and changes to weapon impale classifications, as well as the inclusion of HPL's Call of Cthulhu short story and cosmetic changes.
My take, judging by what I've frequently read online, is that 5.6 is the most popularly played edition.
Of course, 7th Edition is due to be released this year, so all this discussion may be for naught!