I’ve also got both, and I prefer smaller set-ups. I intentionally bought the Syntakt to replace my old M:C, which was my “toy” for bedroom jams and trips… so my Syntakt use is similar - i.e. I’ve not tried to use it with another Elektron, but I pair my A4 with my AR.
I think the Syntakt has a more saturated, more forward overall tone, even before the analog drive is applied. It’s not that the A4 can’t be fizzy, it’s that the Syntakt tends to sound a bit cleaner and dryer around the fuzz. When I first looked into the A4, it seemed to have quite a stiff and brittle tone, but I seem to get “clouds” out of it now I have one. The difference is really subtle and might not be that important.
The ST is quicker to get more of a track going, and as @pselodux said, gives you easy access to a wider range of sounds. The ST’s digital drums and the fm-y and modelled sounds in particular will be difficult to replicate on the A4. The oscillator sync’d and AM’d sounds the A4 can do will be harder on the Syntakt.
I’ve got the A4 mkii. I like taking an individual out or two through pedals and bringing them back in through the external inputs. You can’t add an effects loop with the Syntakt without also using a mixer: but bringing an external source in through its drive is lush.
The difference between Kits and no-kits might be an issue for you. Some people with the Digis get frustrated managing sounds across multiple patterns. Some people with the Analogs get frustrated with managing Kits. I’ve made mistakes with both so I just accept I need to learn the systems and work with them.