I just got the IQ120 and can make a comparison to the IQ110.
The IQ110 did a really good job of temperature control once I got the vent set right. (You want to have the vent set so that if the IQ110 fan is off, the fire will slowly choke out) I learned that if I have the vent number too high, the IQ110 can't prevent enough oxygen from getting sucked in through the unit to kill the fire down to cool it off. The IQ110 did a good job but it sometimes took a long time to ramp up to temp (45 mins) and then would overshoot a little and then stabilize if the vent setting was set right. I was always plus or minus 15degrees F. I run my offset smoker at a vent setting of 1.
Now for the IQ120...
The IQ120 came up to temperature faster without an overshoot. It was amazing to watch it come right up to the dialed in number and stop. I used a Maverick ET-732 to cross-check the temperature and both probes were within 2 degrees F. I really like that I can dial an exact temperature to the single degree as well as see the pit temperature on the IQ120 display. There are a lot of other bells and whistles in the IQ120 but that is for a later date. As far as cooking, the temperature is incredibly stable, I'll have to say more stable than the IQ110 (which is saying a lot). I had a stable temperature for three hours +/- 1 degree F, no exaggeration. The only 6 degree F overshoot I got occurred when I through a couple more pieces of Hickory into the firebox. I could just hear the IQ120 cursing me because it had the fire running perfect and oxygen intake just right. It took the IQ120 about 15 minutes to drop the temp back 6 degrees F to the dialed in temp..AMAZING :)
I was going to say that if you have the IQ110, you don't really need the IQ120. But now that I think about it, the IQ120 is really impressing me with how well it controls everything. If you have neither the IQ110 or 120, I'd say get the IQ120. YMMV.
Thanks everybody for the tips and recipes!!