How to know from specs if one router has better range than other?

Solution 1:

The intensity of a radio signal follows inverse square law. The further the distance, the weaker the signal.

Transmit power = how strong a signal a device can generate. 20dBm > 10dBm, consequently device generating a 20dBm signal transmits further than a device generating a 10dBm signal.

Reception sensitivity = how weak a signal a device can detect. -90dBm < -70dBm, consequently a device detecting a -90dBm signal can detect signals transmitted from a longer distance than a device detecting -70dBm signal.

This imaginary device would both transmit and detect over a longer distance in all spectrums than the one you're referring to:

WiFi Transmission Power 
CE:
<22dBm(2.4 GHz)
<25dBm(5.15 GHz~5.25 GHz)
FCC:
<32dBm(2.4 GHz & 5.15 GHz~5.825 GHz)

11ac VHT20_MCS0:-98dBm
11n HT20_MCS0:-92dBm

The "speed" or transfer rate does not indicate a better range, it refers only to the raw max throughput of the device (clarification: raw throughput between the antenna and the LAN port), in this case in megabits per second. The real-life throughput depends on the signal strength, in practice you can see this by simply the fact that a device near to the WiFi access point can transmit data faster than a device further away. In principle a device capable of 1500Mbps throughput trumps a device capable of 1400Mbps throughput.

However things in WiFi world aren't so clear-cut. The advertised transfer rates always refer to results of testing done in a strictly controlled lab environment. In practice due to various factors present in live production environments (including but not limited to wifi and non-wifi interference, OS/driver capabilites etc.) you will never see this kind of speeds in real life. What you can expect in an ideal environment, near to the AP, without excess interference, is at best somewhere around 2/3 of the advertised max throughput rates.