If you have the ssid the same for 2.4g and 5g it will in most cases detect a stronger signal on the 2.4g and connect there. 802.11ac only runs on the 5g band.
The largest issue is that the 802.11n is a subset of 802.11ac. If it cannot get a good enough signal it will fall back to ones that are the same as 802.11n. You likely could force it to use 80mhz channels and that would mean you will always get 802.11ac but that does not mean you will always get the full speed.