Two services that I use daily to gauge other websites are compete.com and alexa.com. I have both toolbars installed in my firefox browser and it provides me an easy way to get a generalized idea of how much traction a site has. While the numbers provided by these services are far from accurate, they do allow you to for the most part, gauge the web presence of your competitors to your own presence. The reason for this is because if your businesses are competing for the same demographic, chances are the distribution of the toolbars (which these services use as their Neilson box) should theoretically be about the same. In essence you are comparing apples to apples, or massively skewed traffic numbers to similarly skewed traffic numbers.
At the same time, it’s pointless to try and compare different Niche websites with these services because the demographics will be totally different. For example, comparing a tech blog (high probability of readers using alexa or compete toolbars) to a website about crocheting will most likely not provide any real insight into which site has more traffic, but for the purposes of comparing sites in the same industry, these services tend to be correct for the most part.
So in conclusion, take the data you get from these services with a grain of salt, they have their applications and are useful but should not be used to really make any type of important decisions. The only true way you can see which site is getting more traffic, is comparing analytics reports.