HTPC - Mac mini OR iPad + Apple TV?

Airplay (naturally) relies on WLan, which is not as reliable as a cabled Ethernet connection. Especially as you have two streams at the same time: One for the content from the internet (or local media server) to your iPad and then forwarding the stream to the AppleTV. This adds to frequency clogging (next to other WLan devices sending in your neighbourhood).

At my location the 2.4GHz band is - more often than not - only usable with interruptions (=dropouts in streams). 5GHz band is still usable, but the increasing spectrum population is already noticeable.

And while I prefer the better UI usability of the iPad in a living room scenario, it is blocked for the quick inbetween internet lookup during streaming duty. No dealbreaker, but can be annoying. Not even considering the battery issue here.

Thus I'd prefer the Mac (mini) solution.

P.S.: If you're an Amazon prime customer - the FireTV has been offered to those for half the price and thus cheaper than an AppleTV. Airplay to it is said to be usable with an app (can't verify, as my FireTV has yet to arrive).

With all kind of clients (like e.g. Plex, Netflix etc.) available on that, it might be an alternative for you. I've already read comments from people claiming it has completely replaced their AppleTV and they are very satisfied.