Best way to distribute internal apps for small company

We have been supporting a small company's iPad app for about a year now as an Ad Hoc distribution. This was a quick way for us to launch this internal app on a handful of devices.

The issue is that ad hoc builds expire after 90 days, meaning someone always needs to generate a new build for folks to continue using the app.

The app needs to be inside of the company network in order to access the backend web services, so it wouldn't be "review-able" from an App Store perspective.

I know there are several ways to distribute internal apps - some of which were recently introduced. My question is, which method is the best way to go? Enterprise account? App Store (but only for internal folks)?

What's the best way to clearly understand the best way amongst the options?