Did the test. While you lose on RoF, the increased AoE means that the avg dmg done on misses equalises with the RoF of the normal mortar.
Then it's a matter of whether you can use the extended 20 range or not. Whether you want a reliable mortar or an unreliable hard hitter.
As like 2 years ago after nerfs, if you have T2, just get a normal mortar. If you don't, get the 120mm. If you are not playing 1v1 and have absolute domination over your opponent manpower wise, 2x 120mm might be worth.
I had an opponent go 2x 120's. Replay attached.
My opinion of them didn't change much:
The combined cost is roughly equivalent to a howitzer.
The pop cap of 20 decreases income enough (30/min) that player never seemed to have enough manpower to build armor.
One of them got to Vet 3 with 35 kills. For comparison, I had a couple Panzerfusilier squads with more kills and certainly more caps. It wasn't as worthless as I thought, but ultimately the 5-6 squads that they cost (upkeep plus purchase) would've been much more likely to win them the game.
Early in the game while I was worried about the 120's, I got two Leigs. I didn't have any problems keeping the 120's at bay. After awhile, they started hitting my Leigs with Calliopes and a Katy. I probably could've kept them alive but they often had no infantry to shoot and and were about as worthless as the 120's given the scarcity of weapon teams and infantry as targets.