IMHO there are valid arguments for both definitions of console generation.
It makes sense to define the generation by time or to define generation by power.
As long as you make clear what you are saying and which definition you follow, there shouldn´t be a problem.
Personally I define generations by power, thus the Wii U is of the 7th generation.
If someone prefers the definition of generation by time and thinks the Wii U is a 8th generation console, that is fine by me.
Once again, to avoid confusion, you simple should state which definition you follow. That might be a bit annoying but still better than endless debates about a definition (where there is no clear right and wrong).
Sorry for my bad English ;)