Since you seem to be comparing FFT implementations wouldn't it be better to ask for something that can be compared like:
What are the closest 2 tones that can be separated with 3dB of dip between peeks and what's the highest and lowest frequency they can be done at.
Unfortunately the limits are set by the weakest link in the tone generator or the scope.
With Siglent SDG2042x and Keysight DSOX2014A, both enhanced.
Example: 119.99MHz and 120MHz high freq. limited by SDG and resolution by DSO FFT
Example 10 Hz and 10.05 Hz and low, resolution limited by taking too much time to scan
In my case I was interested in the linearity performance of the scope front end and ADC. Also looking at the AWG output with a SA provides a view into how good the AWG signal is. The 2 tone tests was a simple means to show such and why I asked. If you are interested in the limits of the scopes FFT capability, then other tests like you've suggested might be considered.
Honestly I'm impressed by all the scopes behavior, even the low cost "entry' level devices seem to be performing better than I expected.
Now that I've retired (chip design) my plan is to purchase a scope and AWG, then a modest SA for starting a home lab. Have a good initial start with a couple repaired Tek 2465 analog scopes, a couple repaired 34401A DVMs and a new Keysight 34465A DVM (superb DVM).
I realize a scope can't compete with a SA regarding linearity & dynamic range, thus including the SA in the desired equipment list.
Anyway, this has created lots of responses and I wish to thank everyone for jumping in on the fun
BTW the two images show exactly the same screen capture??
Best,