I don't know why the AT&T variant would be different but anything under -110 tends to be very good. Obviously -70 is best but...
At -120 the AT&T variants jump onto HSPA apparently like the Sprint variant does.
I was able to get mine down to -118 and I was still pulling 10 megabits a second down and 5 up with ~60ms latency.
To me that's acceptable... but I have seen people on blogs and forums apply some kind of crazy logic to signal levels... and the fact that Android still has some lingering issues displaying that correctly to a user.
Also note that some apps on the market don't read the LTE signal levels correctly on the HTC One.
They will always display -85 RSRP forever....
In reality unless there is some other issue such as interference a -120 should also get a speedtest done.
It won't be fast but you should not have an issue.
LTE is good to -120 in real world, -140 or so in a lab.