Unfortunately, making that comparison is usually not very easy. Things get that more complicated when the site has a spiky traffic.
Generally, you go for a VPS when you need better control over the environment (want stuff installed or configurations that don't really make sense in a shared hosting environment), site is of a critical enough nature that shared hosting with its possible issues are just not worth the risk, or the shared host says your site is too big for their shared hosting environment. A VPS is not always an upgrade in terms of computing power, and may potentially result in more sluggish performance. Mileage may vary a lot though.