I use a 100, but I know some teams that use the 120. They are very happy with it. A few things to consider for 120 vs. the 100.
The 100 has fairly even temps across shelves, but the bottom and top shelves run hotter. On the 120, the temp is identical everywhere. Is this better? Fast Eddie himself says he prefers the 100 for comps because he likes having a hotter shelf for chicken. You may prefer more even temps though.
The 120 has a firebox outside of the cooking area. On the 100, it's in the cooking chamber. Most FEC100 owners have not had any issues, but there is an increased risk for fire in the 100 vs. the 120, especially if you don't keep it clean.
Teams using the 120 have told me it takes longer to reach operating temp vs. the 100. Not a big deal, but allow it an hour to warm up, vs. about 20 minutes on the 100.
The 120 uses more power when running vs. the 100, due to the convection fan. If you have reliable comp power, not a big deal. If you are planning to run from an inverter/battery setup, it could be a consideration.
The 120 is closer to the ground. More bending over to access the cooker, but less top heavy when moving it around. It's easy to tip a 100 on a trailer ramp if you aren't careful.
So there are pros/cons for each. I don't think you are making a bad choice either way, just depends more on your cooking style. If you are cooking all 4 KCBS meats in a single cooker at the same time, the 100 might give you more flexibility in terms of hotter/cooler shelves. If you are catering (you said you are) and want to load 16 pork butts into it, the 120 will be more consistent across the shelves.
