I agree with those posting various disadvantages to buying the App Store version, and I'd like to point out that one of the advantages to buying the App Store version isn't that big when dealing with Omni. Omni software comes with its own updater code built-in, it's easy to download the software from the Omni website, and you can easily retrieve your licenses from them via their website at
https://store.omnigroup.com/lost/ if you misplace the registration email. Not quite as easy as logging into the App Store and telling it to download another copy of all of your purchased applications, but that convenience (likely to be used very infrequently, whereas some of the advantages might be used daily) hardly outweighs the current disadvantages of purchasing Omni apps from the App Store previously noted.