the biggest different in "smoke on the FE line is the number of burners. A 100 and 300 have one burner, a 500 has two and the 750 has three. Obviously with two or three pots going you'll have that much more "smoke".
I know I've seen some comparison on running them I'll see if I can find it.
You need to price pellets, then figure out which unit and what "average" temp.
For the gassers, will you be running 100% gas (come on now, some do) or will it be a blend on one log and mostly gas or will it be all logs. Then you have to figure out food costs.
Are they all great units. Yes. SP and OH have been around a long time, the FE may be a new unit out there, but it gives you great options. You can mix / match the wood. Try to keep 5 or 6 types of wood logs around.