Always buy OO3 and its sibling products from Omni rather than the App Store. There is a subtle technical difference (bundle code identifier) which prevents the App Store version from running some of the Applescripts on this site, and the options for upgrades and discounts are more flexible in direct purchase from Omni.

You may also want to take a view of the fact that Apple (which now vies with Exxon for the title of most voracious capital accumulation machine on the planet), takes a 30% cut if you buy OmniOutliner through the App store.

My personal view is that that 30% is more likely to feed back into the product, and its future development, if you buy from Omni.