When I want to compare to solid like white/black background I just use Y. When I want to compare colors I have had decent enough results with this simple formula (conveniently in RGB for most programing tasks):

http://www.compuphase.com/cmetric.htm

I think it's pretty telling that CIE and others have made a number of revisions to Lab color distance formulas over the years to the point of them having constants you tweak. There's no perfect out there yet, so I use a good enough that is simple to compute.