Display update rate does not correlate to waveform update rate.
The older and more expensive SDS2354X HD has a higher display update rate than the SDS1204X HD.
In theory, one might assume that the "optimal" display update rate would be the actual physical display update rate (typically 60Hz, or 16.7ms intervals), but UX specialists may disagree; with something around 25fps considered a good balance (between visual and data bandwidth needed).
To my eyes, the SDS1204X HD definitely looks less than 20 display updates per second, because every second or third frame of the 25fps video shows the same waveform. The numeric display updates only occur at about 8fps or so, because they stay unchanged for about three consecutive 25fps frames.
In comparison, every frame on the SDS2354X HD at 25fps shows a different waveform. Thus, there is definitely a display update rate difference between the two.
We'd need a higher framerate video of the two, side-by-side, to evaluate/estimate the actual frame rates. For accurate numbers, something like 120fps - 180fps video would be needed, although colors and resolution would not matter as long as the waveform display updates are detectable.