The AC routers are much faster, with a nominal speed of 1.3Gigabits. That's much faster than internet bandwidth, so it won't be useful for a faster internet connection, but it will make a difference with inter-network transfers. Transferring files over the Wi-Fi network will make a big difference. The signal won't extend as far, however, because it uses the 5Ghz band instead of 2.4Ghz. This is actually touted as a benefit because the 2.4Ghz band is so crowded.
The 2.4Ghz band, used by current routers, is an unregulated band with a lot of things that run on it. This creates congestion and interference. It also aims the signal to the device you're using, called "beam forming". This reduces congestion and gives a much better signal. This new Wi-Fi standard is aimed at reducing the channel congestion within the coverage area, which has become a real problem in Wi-Fi.
This new standard is at least a year off, though, and Leo doesn't recommend this new router yet. He doesn't think most people really need this increased speed, and while it does help with congestion, a good quality 802.11n router will do that too. Leo's problem with the Buffalo is that it supports an encryption technology called WPS, which is very insecure. Those issues combined with the higher prices of these new routers make it impractical as of now.