@RTMC 

In theory, the higher db on the 670 should give you a bit more power to blast through the walls / interference.    However if honest, the difference is likely to be so little it would hardly be noticed in reality

The 670 is a newer model device, so personally, if you can get it for just a few $$ more than the 660, likely a good buy!

More power is not necessarily a good thing. Smartphones and most laptops max out at around 23dBm, so while those devices will see the AP, they won't be able to transmit back to it. This results in a poor user experience since phones and laptops hang on to a signal that isn't usable instead of switching to mobile data or roaming. If many thick walls are the issue, more APs with lower TX power is a better solution.