Feature comparisons require a lot of work and can be difficult at times. This is especially true for the trigger suites:
Siglent Pattern trigger is more powerful and contains a time condition for the duration. Hence easily replaces the Rigol Duration trigger.
Likewise, Siglent Interval trigger (not even mentioned in the table) can replace the Rigol Delay trigger.
Siglent provides not only holdoff by time, but also holdoff by event (an unique LeCroy feature), which can not only replace the Nth edge trigger, but also implicitly provides Nth pulse, Nth slope, Nth pattern/duration, Nth interval/delay. All these can be extremely handy and aren't available on competing instruments.
So the only trigger that is currently missing is the Setup/Hold trigger – but this has already been added for the SDS5000X with the 0.9.1 firmware, together with a genuine Nth edge trigger and an explicit Delay trigger. So it is only a matter of time when it will be available for the SDS2000X+ as well.
Decoders: The Siglent decoders are full duplex, i.e. can handle protocols that use individual lines for RX/TX. So you can decode an UART and SPI connection at the same time, which means four data channels have to be processed in parallel. You cannot decode four I2C connections simultaneously though.
Parallel decoding is of course available, just not on analog channels. With the MSO option you get parallel decoding as a part of the MSO, not the decoding suite.
Math:
A high number of math operators might look impressive, yet the question remains whether they are really needed or even useful.
Siglent is aware that there are some operators missing, as they have a team of beta testers and consultants, from standard industries up to the high end industry. And in these professional fields, the requirements (and priorities) look a bit different. For instance I'm not aware of any request for Boolean algebra in the math menu.
Once again, the SDS5000X with current firmware can be used as a hint on the possible future improvements for the SDS2000X:
Sign, Abs, Exp, Log, Identity and Negate.
You don't absolutely need Ax + B as long as you've got a formula editor.
But yes, this is about all what engineers have been expecting even from a midrange DSO like the SDS5000X.
Mind you that in the formula editor, the Sign function can of course be "abused" to do Boolean algebra – should you really need it.
A filter package is under development for the SDS5000X – I have no clue if this will become available for the DS2000X+ as well. And this is not because Siglent wants to cripple the lower end model, but there might be insufficient hardware (read: FPGA) resources left – let's wait and see…