OK take this for what it's worth. As per SmallNetBuilder reviews, they've changed their format a little between the time that the WRT1900AC and R7800 were reviewed. However overall, the throughput numbers show that the Netgear R7800 is faster. Unfortunately that doesn't tell the whole story. Since they seem to use averaged throughput for all locations (physical) tested, it's obvious that the faster router is going to have faster average results, but this doesn't highlight what happens as range increases.
If you look at the test chamber results, things are a bit mixed as far as throughput. First by attenuation then by frequency. I'll try to explain and hopefully I can do it in a way that's not confusing.
First off at lower attenuation, on both bands, the R7800 is faster. On the 2.4GHz band, the R7800 performs better at all attenuation values. Since this is the band that will penetrate physical barriers the best, this is the one that will give you the best overall range.
However at 5GHz things are different. Again since the R7800 is a faster router, up to -15dB the R7800 is faster. However from -18dB onward, the WRT1900AC is faster and can maintain a connection up to -45dB where the R7800 can only maintain a connection up to -39dB. These results are for download.
Now the 5GHz upload is similar. The R7800 is faster up to -15dB. From -18dB onwards, the WRT1900AC is faster and it can maintain a link up to -45dB where again the R7800 only maintains a link to -39dB.
Now attenuation doesn't always translate into greater range. Since the method of attenuation is artificial (not real world), you need to take it with a grain of salt. However I believe you can loosely equate attenuation in these tests to simulated distance / obstructions.
So to sum it up, the R7800 looks like it will overall provide better coverage on the 2.4GHz band. It also provided the best throughput on the 2.4GHz band at all "ranges". The 5GHz band is a mixed bag though. At "close" range the R7800 is going to outperform the WRT1900AC (by quite a bit at very close range), but once you start to get "further" away from the router, the WRT1900AC will have better throughput and maintain a link beyond where the R7800 can. Since coverage is your primary interest, the R7800 will provide the best coverage overall due to it's 2.4GHz band, but this will be at 802.11n as 802.11ac is only on the 5GHz band. If you want a mix of coverage and throughput, then you need to look to the 5GHz band. In this situation the WRT1900AC is going to likely have better coverage and will have better throughput past a certain "range".