I think some folks covered it well as 802.11ac more often than not is much faster than 80211n under the right conditions.
Unlike what one person said, all connections speed on WiFI are NOT determined by the slowest device. It is a matter of the router and the setup related. As example, using a WiFi ac router while someone in the net room is using an android device and someone has an iPad watching Netflix (no joke). All of us are as if we are the only one's on the net as there is no issue of service. The iPad and the android device are 802.11n and I am using on my laptop ac. One of the n devices is at 2.4 ghrz and the is at 5 ghrtz as am I (as that is where ac does its 'magic.'
I'll just say if you decide to get a new router, why not explore non-Apple routers as well. This is not to say that Apple's router is bad, but at the same price there are better WiFi routers out there and some do amazing with QoS (quality of service) as a feature where YOU decide how to allocate your internet connection. (Perhaps in my case the Neflix user might have the most given it is a function of streaming etc.) Smallnetbuilder site is a good place to start.
As for me, I have used various makes of routers over the years and the key is to get a good actor that has stable firmware and admin mode that you feel comfortable. Makes like Linksys, Apple, Netgear, Asus and a couple of others seem to all have some viable offerings (yes I did mention Apple only because some find it friendly and don't mind giving up some features that others provide that can be very useful including some security facets).
Last item to toss out here - With decent ac routers, they do "n better than n routers" meaning that 80211n performs better with these routers than with routers that don't do ac but only n. This is not an opinion but has been tested and metrics to go along with it.
In your shoes, I'd stick with your present set up and see how your ATV purchase performs. If you have other devices that are WiFi, then time to do your homework and select a new router.
This is the typical household here - 2 desktops Windows, 1 Windows laptop, my rMBP, 2 iphones, 1 android phone, Nvidia Shield TV (like ATV but does a bit more things) all on WiFi. As I positioned the WiFi router properly, everyone can be on at the same time and the only "choke" is due to the Internet connection speed itself at the modem (how much up and how much down by our provider).