The lighter bars to the right are the x64 version of the respective OS
I have been using Windows 8-DP x64 exclusively for about the last two to three months and it is even more secure than Windows 7 SP1 x64. Of course once we start to see Windows 8 RTM on computers with Unified Extensible Firmware Interface (EUFI) sometime late this year https://en.wikipedia.org/wiki/Unified_Extensible_Firmware_Interface
those computers will be even safer.
Source: Microsoft Security Intelligence Report
http://www.microsoft.com/security/sir/keyfindings/default.aspx#!section_4_2