Velocicoaster will always have the theming advantage over IG, but the actual ride on IG could easily be superior.
This is why all of people's coaster rankings are so subjective. How do you compare a highly themed ride experience (but a mediocre coaster) to a great coaster with minimal or no theme elements. Not that I'm saying Velocicoaster is mediocre, but when you look objectively at rides like Big Thunder or Everest (which are mediocre to pretty lousy but well themed) how do you compare them?
I think for most of the general public Velocicoaster would win against IG in a head to head competition, but for coaster fans IG would be the choice.