This one is honestly quite a tough one! As a previous DT owner I can she’d SOME light, but there are much more qualified people here who can give you better synopsis of each. Here we go!
Digitakt and Digitone don’t have song modes so you would need to find a way to sequence songs out of them externally or just get comfortable with pattern chaining/juggling. Each also only has 2 outputs and Overbridge is nowhere in sight so everything you want to record into Ableton (if you plan on doing proper mixing, that is) will have to be recording one track at a time in sections then arranged later. This kinda kills the performance vibe they give you and gets more tedious the more complex your tracks become. Neither have external storage either so you’re limited to the +drive and transfer apps for any samples. Ease of use though is incredibly and both sound amazing. They also have better sounding FX than the OT (IMO) but not nearly as many per track. 1 lfo per track can be a bit frustrating at times for example. The DT also recent got a master Compressor which helps tremendously with mixing within itself. Haven’t used or studied the Digitone much so I don’t have too much to say on it aside from the demos I’ve seen sounding just beatiful.
Octatrack has an arrangement mode which can definitely aid in the creation of full songs OTB. Where it lacks is that it doesn’t make sound on it’s own (duh, it’s a sampler) and it can be quite a task to learn if you’re new to Elektron. It has 4 inputs and 4 outputs if I’m not mistaken, stereo sampling, external media via CF cards, more FX (although no better FX than DT and DN) and many, many more features which makes it a dynamic performance beast. Imagine the Digitakt as the baby-brother-drum-machine version of the Octatrack. They can do similar things, but the Octatrack is much more advanced and versatile.
If you don’t need the extra outputs/mixing capabilities, a DT and DN combo will be a simple step in to the Elektron world and will give you plenty of creative abilities. And once Overbridge is out you’ll have even more functionalities and can sequence in your DAW, although this takes away from the OTB feeling.
The Octrack is something that you really have to give time and learn inside and out, step by step. It could take years even. But it’s incredibly worth it from what OT owners have said. There’s a few people who have done a Metronome challenge with the OT where their only sample source is the OT’s own metronome and they’ve created full songs from it. One dude even made it sound like a saxophone which is just nuts but it was great. The ways you can mangle samples is just unrivaled. I can’t wait to actually get one myself, but I just don’t have that kind of bread or gear worth trading for it (unless someone wants a Push 2 for it lmao.)
TL;DR. Digitakt and Digitone for easeof use and quick creation, OT for a unrivaled Sonic mangling and song modes but steeper/longer learning curve. Also take a look into the Analog Four and Keys. I’m pretty sure one of them has a song mode as well.