Accelerometer; What is the dif between xGravity and xInstant?

As the questions states, when using the accelerometer what is the dif between xGravity and xInstant?

For specifics: I am making an app to balance an RC Jet’s Electric Ducted Fan. It needs to capture very slight vibrations from a small (2.5 inch) spinning (40,000 rpm) plastic fan. I’m not sure weather I should use differences in gravity or the instant results to figure out weather it is vibrating more or less…

Thanks!

Gullie