The above isn't entirely correct.
Both N and AC can use either 2.4ghz or 5ghz. As with all wifi standards, 2.4ghz allows better wall penetration, but slower speeds due to a variety of reasons. Also as with all wifi standards, 5ghz allows faster speeds but less wall penetration. Basically, in most homes, this means 5ghz has less range.
However, AC standardized some technologies that overcome a bit the 5ghz penetration problem. In particular, beamforming (which has existed for a while, but wasn't part of the standard before AC) increases the range of a 5ghz signal.
From the testing I have read, range in a regular home will still be best when using the 2.4ghz bands. However, the maximum speed is much much faster on 5ghz - which is why most people configure it to use those bands.
As for the OPs question - getting a new iPad will not help your range at all. With wireless, it takes two to tango. You would need both a new AC router and a new iPad with AC to take advantage of AC. Further, you would need to ensure that all your G or N devices either connect to a different SSID or are also upgraded to AC. If a slower G or N device connects to your new AC router, the router will step everyone down to G or N. In general, routers will use the lowest common denominator technology. This means they will use whatever the oldest or slowest of everything connected.