That depends ENTIRELY on what you are doing, because some applications will benefit more from having more memory and others will benefit more from the additional responsiveness of a dual channel configuration with it's doubled bandwidth.
In general the 8+4 will likely be the preferred configuration because of there simply BEING more RAM, plus the consideration that on most modern systems with a configuration like that it is probable that it will run in a "Flex" type mode, where 4+4GB will run in dual channel and the remaining 4GB will run in single channel.
Either configuration is viable though, and it will depend largely on what you primarily use this machine FOR?