Yes a separate logic analyzer (in addition to a (DSO) scope) is good to have anyhow.
While a DSO scope is not a replacement for a logic analyzer, the serial decoding and triggering features on the SDS1104X-E are significantly better and more useful than on the DS1054Z.
In Europe the Siglent SDS1104X-E is 31% more expensive than the 3-year old Rigol DS1054Z, That 31% gets you a scope with significantly better specs and features, is more modern and more future-proof.
A 4-channel scope with decent serial (SPI, I2C etc.) decoding and triggering (like SDS1104X-E) has the advantage that you can correlate these serial signals with other - both analog and digital - signals.
When investing in a scope for the longer run:
For better specs, more versatility and better usability, the extra 31% for the SDS1104X-E over the DS1054Z will be well spent. (But if you are never going to use its better specs and additional features (which I doubt) then better save those 31% for something else.)