Cat health Plz read details?

I have an indoor cat and he's been throwing up his hard kitty food. We took him to petland vet hospital and they gave us a so called hills diet prescription for hard kitty food. But we are wondering should a vomiting cat be put on sensitive hard kitty food like or gastrointestinal food? Or is it gastrointestinal same as sensitive stomach? And in reality is it better for him to be on the prescription after they are sales bots at petland and we feel that any sensitive stomach brand is the same as the prescription which isn't really a prescription at all.