I have a Q320 and recently got a Q120 to use as my main grill, the reason being that I usually am only grilling for one or two people. The Q320 works for one or two as well as larger groups, but it can be a bit overpowered when grilling only one or two pieces of salmon or burgers. I'm still keeping the excellent Q320 for when I need a larger grill.
I find the Q grills work better when they are closer to 50% full with food or more. When they are nearly empty, like a Q320 with just two burgers on, they can create more of an oven effect and its easy to overcook the food, and the flavor isn't quite a good. However, with 2.25 people and frequent guests, you may want to consider the Q320 for the flexibility though. The Q120 will work, but you may have to do two shifts on the grill with larger groups.
If you do use a Q120 as your main grill and connect to a 20 lb propane tank, make sure to get a propane filter to prevent the odorant that liquifies in the hose from ruining your regulator. I use the Mr. Heater propane filter which is designed to prevent this same issue on propane heaters.