Rigol is a known quantity with pretty good quality (for the price), with lots of reviews and experiences available. Atten not so much. This alone would tilt the scale in the direction of Rigol for me. The fact that they were sued successfully by Rigol for copying their design doesn't help their reputation, either.
There seems to be some confusion about the memory depth. The features list specifies 4k for the ADS1000 series and 2M for the ADS1000M series. The table below lists 1M/2M for all ADS1000CA models. In the datasheet (catalog), it specifies 40kpts memory depth, max recording length 6M, and 1M/2M in the table. Specs at the end list 20k/40k again. Reminds me of the Uni-T scope.
No manual available for download. Attempting to download the ADS1000 catalog appears to return a Chinese 404. The Rigol documentation is not great in my opinion, but the Atten documentation may be even worse if it was written by the same person as the Atten website.
The 1.2Mohm input impedance with AC coupling below 100mV/div seems a bit dodgy (would introduce 20% error with a 10x probe at lower frequencies). Noise spec doesn't seem great, although I can't find a similar spec for the Rigol scope, so I'm not sure how it compares. 40% tolerance for the 20MHz BW limit seems pretty bad.
Most of the possible issues will not be reflected in the datasheet: if it actually meets the specs (not too convinced about that, even for the Rigol), the UI, responsiveness, durability, support (not Rigol's strong point either). But it may actually be better than the Rigol, I don't think anyone has done a review yet.