Users who trust direct relationship with you, will download directly, enjoy better prices (30% commission omitted) and rapid updates.
At the same time you will not lose a whole marketshare of users for which AppStore is the "go-to" address for buying software.
You can then skill up your SEO capabilities and optimize inbound traffic to your preferred platform whatsoever.
If AppStore policy forbid having a parallel distribution platform, then publish the same app with two different names.