I would suggest to always select the 6k software based on safe harbour.
http://www.cisco.com/en/US/customer/netsol/ns504/networking_solutions_program_category_home.html
The current recommnedation for native is 12.1(26)E2
and the recommendation for hybrid is 7.6(12) and 12.1(26)E2.
always make sure to read the release notes for system requirements or any specific bugs before upgrading.
HTH.
Salman Z.