The knobs of the SDS2000X Plus make a somewhat cheap impression on the pictures - or is that just deceptive ?
That would be least of my worries. Be aware that Siglent (and Rigol scopes) typically only decode what is on-screen. If you want more packets on screen then you have to resort to zoom mode and thus give up screen real-estate in a situation where the screen is already cramped. On top of that Siglent scopes only record enough data to fit the (zoom) screen (unless they have fixed that in the meantime) regardless of the memory depth setting. The latter can be annoying because you have to go back & forth between settings if you need to repeat the same measurement several times.
The R&S scopes OTOH don't have these limitations; it will always have decoded data for the entire memory depth you selected. If working with protocol decoding is important I'd go for the R&S scope.
And you don't see 1/10 of the memory in a R&S a limitation ?
When with the SDS2000X Plus you can set a timebase with memory depth to 200 Mpts, capture and examine the complete record in detail.
But likely the limit for the number of decoded frames is far exceeded with such a long record. Something else to look for when using protocol decoding is the oversampling factor. This can depend on the actual memory depth used to speed up processing. So in the end the question is: how useful is the long memory record in reality where it comes to protocol decoding? Better do some testing to back up the claim 200Mpts is better. It may not be the case at all.
Decoding is not the only use of deep memory and yes your question about packets is clearly listed in the SDS2000X Plus datasheet:
https://int.siglent.com/u_file/document/SDS2000X%20Plus_Datasheet_DS0102XP_E01A.pdfI
2C Decoded frames (Max.) 2,000
SPI Decoded frames (Max.) 15,000
UART Decoded frames (Max.) 15,000
CAN Decoded frames (Max.) 2,000
LIN Decoded frames (Max.) 3,000
CAN FD Decoded frames (Max.) 1,000
FlexRay Decoded frames (Max.) 1,000
MIL-STD-1553B Decoded frames (Max.) 10,000
The R&S RTM3004 I tested didn't had any trouble recording 32000 messages for example.
OK so you're not comparing apples with apples.
RTM3004 is a 5 GSa/s DSO with 80 Mpts max memory and is in another totally different price class to SDS2000X Plus DSO's.
OTOH the OP was looking at RTB2000 which only has 10 Mpts mem depth so could well be more limited in the # of packets it can display.