ExtraStats and SiteStats are completely independent.
ExtraStats collects extra information about HIT data for your links - so that you can display lists by hits to links (most popular links today, most popular this week, most popular categories (by links hits), etc.).
SiteStats collects information about pageviews to your site. You can show the most popular pages, most popular detail pages, most popular categories etc. You can group pages however you want depending on the lists that you want to show.
There are plenty of globals contained within the plugins so you shouldn't need to use your own globals - but obviously you can if you want to. You just add tags to your pages where you want to show the lists, graphs or stats. I'm happy to add features to the plugins (as time permits) if anyone has any requests.
Laura.
The UK High Street