WMT P/E is 20, Costco P/E is 22, not a whole lot of difference, and if one clocks this link, despite the fact WMT has been very firm on wages and benifits to the point they have taken very massive PR hits, their share price has gone down 20%, while Costco has gone up 25%.
http://www.google.com/search?oi=stock&q=stocks:WMT&prev=/search%3Fq%3Dwmt%26hl%3Den%26lr%3D
WM's stock price has been in a muddle for a few years now, that's common knowledge.