To be fair it depends on you as a person and how you work.
My honest answer based upon my usage is 4 channels is the way to go.
If you think of a customer who brings a car in for lack of power, lets say a Common Rail Diesel engine, No Fault Codes present, where do you go?
Well you could connect the scope as follows:-
Channel A: Digital MAF
Channel B: Fuel Rail Pressure
Channel C: Boost Pressure Sensor
Channel D IMV Valve.
In one simple WOT test you get a picture of the Intake/Fuel system which potentially allows you to see all the parameters which could cause loss of power in one hit. Now we know these may not be the issue all the time, but its a quick way of seeing whats going on.
With 2 Channels you would probably want to run a combination of multiple tests which takes longer in the long run potentially making the job last longer then it needs to.
In reference to our customers, most customers who purchase a 2-Channel Scope always come back for the 4-Channel model and by doing this costs a lot more long term, so if you can budget the 4423 or one of the 4 Channel kits then this really is the way to go.
I hope this helps to answer your question.
Lee - Automotive Technical Specialist