Preventative Represention

Recently, I saw a sign by the elevators indicating that it's undergoing preventive maintenance.

It bothered me because I thought that the word should have been preventative.

But upon reflection, preventive made sense as the root word is prevent, not preventate.

Asking around, it was consensus that both were real words. This prompted me to googlefight them. The result I got was (YMMV) 7.06M preventative and 5.36M preventive. This led me to further google preventative.

Seems that most scholarly discussions (particularly medical) seems to believe that preventive was the correct form and preventative should be discouraged (opposite of googlefight result).

However, trying to formulate sentences, I think the following rule of thumb works for me (I made it up): When using as a noun, use preventative, but when using it as a adverb/adjective, use preventive

As in:
Getting a flu shot is a good preventative against the flu

Getting a flu shot is a good preventive measure against the flu

Idle thought: why is a congress person a representative instead of representive? When they are supposed to represent us (Unless you're Al Sharpton, then congress people representate us)