sam222 - As you've phrased your question, it sounds like it's largely about Google AdWords landing page setup.
There is an SEO aspect to your question, though, about the possibility of duplicate content if example.com and example.com?g=code have the same content. For that reason, it's best to make your AdWords URLs to be a specific landing pages for selected keywords, in this form...
...and to block them from spidering or indexing, either with robots.txt, or with the meta robots noindex tag.
I would not direct AdWords to your home page, because you clearly want that available to Googlebot for organic indexing... so you wouldn't want to block it.