I overlaid the Rigol waveform on the Siglent waveform in a single image. As you can see they're identical when you view the wave with the same settings.
I would say that this scope has a very serious issue when sharing the A/D converter with more than one channel.
There's more than one channel enabled in the image above...
The Siglent behaves much better.
There's a major difference: The Rigol drops down to 250Ms/sec when you turn on the third channel, the Siglent doesn't (because it's only 2 channels).
At 5ns per division there's 1.25 samples per division on screen. I'm not sure how they're aligning the trigger point so exactly but it looks like they're not offsetting the rest of the wave to match. The "jitter" looks to be about 1/1.25 of a square.
Identical? This is total false. If you use identical input then they can compare. Do these image have exactly same signal?
Then
There's a major difference: The Rigol drops down to 250Ms/sec when you turn on the third channel, the Siglent doesn't (because it's only 2 channels).
Reason is not because Siglent have only two channel. Root reason is construction.
When max 1GSa/s Siglent have 4 channel on it do not drop to 250MSa/s.
Rigol construction is cheapest possible:
1ADC1MEM/4
Siglent 4 channel construction is semi cheap:
1ADC1MEM/2 + 1ADC1MEM/2
If compare Siglent 100MHz 4 channel model and Rigol 100MHz 4 channel model:
Rigol 1Channel on, 1GSa/s max mem 24M (officially without option 12M)
Rigol 2Channel on, 500MSa/s 12M both channels. (without option 6M)
Rigol 3Channels on, 250MSa/s 6M all channels (without option 3M)
Rigol 4Channel on, 250MSa/s 6M all channels. (without option 3M)
Siglent 1Channel on, 1GSa/s max mem 14M
Siglent 2Channel on, 1GSa/s max mem 14M both channels
Siglent 4Channel on, 500MSa/s max mem 7M for all channels.
Some other oscilloscope may have other construction
example 1ADC1MEM + 1ADC1MEM + 1ADC1MEM + 1ADC1MEM
this kind of oscilloscope have always same sampling speed and memory independent of how many channels is simultaneously in use. This kind of construction is of course most expensive (And best).