Thanks everyone, this is all really interesting and useful.
A couple of things I’m not sure about which you may be able to help with:
Sample Management:
With both machines, how does sample management work? For example, the CF card for the Octa and +Drive for the Rytm can hold plenty of samples for me. How are these accessed during real-world track building use? Once you’re in a a project, with it’s 128 kits, can you still access and load in other samples from the main drive? Or are you limited to what you’ve already pre-loaded?
I’ve seen some info on Strom which looks like you can just load samples in and out of the Rytm on the fly, does anyone have any experience working like this? I don’t have an iPad, but I may have down the line.
Really, access to all my samples is important, because when I start building a track I never know where it’s going to go, or what I’m going to need.
FX:
For both machines: Are the master send FX and track FX different? And can you EQ either of the FX?
How do the reverbs compare?
I’ll probably never use more than reverb, chorus and delay anyway. I’m not into bit crushing or anything like that. Plus I can get a pedal when I need to.
Tracks:
On the Octa, there are 8 stereo tracks. Straightforward. But the Rytm has 8 voices, but 12 drum tracks - how does this work? Are there 12 tracks sequencing for those 8 voices? Or is this 8 internal tracks and 4 for external gear?
Output/Input:
Both machines: If I use an output to go to an FX unit and bring it back in, does this use up one of the tracks, or is this an extra monitoring route?
Sample Playback:
Apart from the mono/stereo thing, how do the engines differ for sample playback with maybe some filter and FX work etc. Are the basic sequencing engines the same. In what way does the Octa outstrip the Rytm for samples? Sampling inbox and resampling aside, can you give me some examples of sample based stuff that you can do on the Octa that isn’t possible on the Rytm? I don’t work with loops or need tempo matching/time stretching/chopping etc… more one-shot stuff, but I’d still like to be able to arrange musically, so for example to program in a bass sequence or whatever.
Really, the Rytm’s analog drums are an extra for me, I probably won’t use them much, except maybe the kick sometimes to beef things up. I have good drum samples that I’m happy with. My main focus is working with samples of all types, percussion, synths, atmospheres, found sound, pianos, voices, etc. even if I have to import them from somewhere else rather than actually sampling them on the machine.
As long as I can have easy access to my samples when I need them and have the ability to mould them and process them in a subtle but creative way and then sequence them as you would on an Octatrack, I’ll be happy… although stereo would ace, as would be the ability to actually sample in the box.
Honestly I could happily make music with a Korg Volca Sample if it had longer pattern lengths, easy sample management and an easy way to track-out… but that’s why I’m looking at Elektrons 
So If I can do that that on the Rytm, I think it would be a better option as you also get nice analogue filters and Overbridge - which could prove invaluable in tracking out.