It all depends what you do.
If you do lot of troubleshooting (like repair), 1 channel is enough most of the time... You probe around the circuit looking for expected waveforms.. Mentally you are doing one thing at a time... And sometimes you want to time correlate that with other signal (drive, clock..) so 2nd channel...
For that kind of work, 2 ch and more bandwidth is better... Also, doing that, you twiddle with settings more.. So people doing that kind of work will complain more if scope is not "responsive", and faster UI will be important to them.
If you do development, than 4ch is not even enough sometimes... Switching power supplies, solar chargers, mixed signal.... They all have many parts of the circuit that have to work together, with timing and all kinds of details.. Not to mention looking at voltage and current at the same time...
And also, while doing that, you don't twiddle with settings and buttons that much..
Most of the time you twiddle with circuit or firmware, and watch for the changes on the screen..
While I agree that DS1104Z GUI is not that responsive moving vertical traces as some other scopes, it never was a problem to me...
I do things slowly and deliberately..Slowest thing is always me...
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Also, I only see slow response on vertical channel position settings.. For other settings, I didn't notice any particular sluggishness..
And all of that has nothing to do with waveform screen updates.. Those are fast and realtime.. In fact, GUI is slower because it was given less priority that waveform display..
My experience is that I usually use 1ch for quick checks, or 3 or 4 for troubleshooting and development... So for me 4 ch is a must..
As somebody mentioned, if that new Siglent SDS1202X-E was in fact SDS1204X-E, well,
that would have been interesting to me.. Provided it's reasonably bug free and such... Even for a few bucks more..