ATV3 no longer gets software updates. If all you want is AirPlay, this could be ok. But, AirPlay can be slow, ATV4 gives you more options for the future. If you aren't planning to load it up with many tv and gaming apps, the 32GB should work well for you long term.
If your only apple device is a MacBook, no iOS devices, and you don't see an iPhone or iPad in your future, you're better off connecting via HDMI,as it's faster, maybe get a longer cable.