What is the difference between In House versus Ad Hoc distribution for Enterprise apps?
Ad-Hoc releases have to be provisioned to specific devices. Your account allows you to register up to 100 devices.
In-House is for distribution to any company device.
So in most cases In-House can serve all your needs. But in the event that you wanted someone who was not part of your company to test something or you wanted to restrict distribution to specific devices you could use the ad-hoc provisioning.
Update: Ad-Hoc now allows up to 100 of each type of device. ie. 100 iPhones, 100 iPads, 100 Apple TVs, etc