After Song Mode was released, I’m considering moving to the “Digi Trinity” (Syntakt/Digitone/Digitakt) as the core of my studio.
When composing with multiple Digi units, how do you usually sequence them? Do you use one unit (Digitakt/Syntakt) as the master sequencer, or do you sequence the respective elements of the song on each unit and just send clock from the master unit to the others?
I’m pretty sure that 95% of the users will do the sequencing on each unit seperately. If you don’t do that, you can only sequence midi notes and the Elektron sequencers are really all about the advanced features.
Yes, one is the master, but just to send tempo, transport and pattern changes to the others.
I agree with the statements above and this is how I work with them too. The upside of sequencing via midi being the addition of an extra lfo ( OT has three…) or using OT’s MIDI arpeggiator. But we aren’t talking about Octatrack, are we…?
With the addition of Song Mode, nothing will change. Digitakt will still be the one sending Clock, Transport and PC to the others. Each box will sequence its own Tracks and DT will sequence my other non Elektron Synths. However, I will surely have the same Song structure on each box (the Mutes option is a great addition and will allow me to concentrate on other things like playing Keys and tweaking knobs).
Would you consider the DT better suited to sequencing non-Elektron synths than the ST?
(I liked the repeat feature of the ST when I tried it out… does it work on MIDI tracks?)
Yes as DT has 8 dedicated MIDI Tracks. I’m not sure I would want to concede any Tracks on the ST as I often layer them.
Do you mean the Modifiers? I never use them and wish they were MIDI Tracks instead or at least have the option to switch from Modifier to MIDI Track.
If you had the choice to buy the DT or ST (but not both), would it be the DT?
(I already have a DN, A4, and AR.)
It depends on the number of non-Elektron Synths that I/you have. DN can already take 4 Synths with 8-note polyphony per Track.
How many notes can the DT have per-trig on a MIDI track in comparison?
I had originally started to use AF to send clock and start stop to DT and DN and use its song mode to also control pattern changes. This required me to set up patterns on each device according to the song structure.
I no longer use the AF but the OT as the brain, which could be even more powerful with its arranger. But I have started to only send start/stop and clock signals to DN and DT. It felt to cumbersome and limiting to me, since it would always require thinking ahead when coming up with sketches and placing patterns accordingly on each device. Or a lot of cleanup after an idea has finished.
That being said, now that DN and DT also have song mode I think I will take the time to program songs of actually finished songs into each devices pattern banks, so I can record them more easily and maybe focus performance on turning knobs instead of mostly muting/unmuting.
So basically I would recommend a mixed approach with multiple options. However, if you are the person that comes up with a fixed structure of a song spanning across the three devices quite early on, I would consider sending pattern changes from one of the digis to the other ones. With loop mode, you will still be flexible enough to leave the planned road.
how do you sequence them?
Sheer willpower! 
(and recently with a little help from song mode)
All other desires being equal, were I in your situation I would opt for the DT over the ST. Why? Because with the DN, AR and A4 already in your arsenal, you are already capable of covering most of the ST’s sonic territory. A dedicated sampling drum machine like the DT would serve you well, with its focus on sampled sounds and its 8 dedicated MIDI tracks.
I LOVE my tiny Trinity (DT/DN/ST), but those are my ONLY Elektrons. Even so, if I only had the DN, I would probably still lean toward the DT as a “next” box. You with the AR and A4 already, that would push me even harder into the DT’s arms.
That said, because I don’t own those higher-end units, I LOVE LOVE LOVE what the Syntakt brings to the table for me.
This is just my opinion, YMMV.
I sequence the DN from the DT because:
Single-handedly representing the 5% here 
Not completely true, you can p-lock any CC message you want, or modulate them with the LFO’s. In the modulation setup on DN, you can configure AT/PB/BC/MW to map to any parameter you like and p-lock these from the DT sequencer too.
It’s still limited compared to sequencing on the DN device itself, but you can still do a lot and just use one sequencer (with song mode nowadays).