I have the Syntakt and recently bought an Analog Keys (used). I think you would have a hard time making patterns as quickly on the AK, especially at first. You will not have as “immediate” of a workflow, but the AK is an awesome sounding machine and if you’re not going to feel limited by 4 voices, it can do things ST can’t.
The Analog Keys has individual outputs for each of the 4 voices, which is the same as the A4Mk2 but an upgrade from the original A4 which only had stereo outs. It also has a joystick mapped to pitch, modulation and breath control, and an aftertouch keybed, which are quite nice and playable. The frequency response is much more bass heavy than the original A4 (arguably too much low end, at least for some).
The screen is much less pleasant to look at compared with the modern Elektron screens. You will get used to it, but at first, expect it to be a downgrade. The screens are also prone to going bad eventually.
You will likely miss some of the visual indicators of ST’s interface, and you will almost certainly miss the encoders. The encoders on the Mk1 and AK are much less responsive/ sensitive and can be a bit of a pain.
Getting a “new old stock” AK is cool, but to me, $1300 is a tad high, even brand new. On the used market, if you keep your eyes peeled I feel certain you could get one in good condition with box and paperwork for between $900 and $1k. I got lucky (and patiently waited and watched listings) and got mine in good shape for approx. half of what you’re considering paying for a new one. Just food for thought on the price.
I hope I don’t seem too negative about it - I adore the AK and ST and plan to keep both. But if I had to choose, I’d keep the Syntakt because I find it more immediate for sitting down and making music with. And I don’t think I’d personally spend $1300 on a new AK.
Edit:
I need to look into this. The AK has a bipolar parameter for overdrive in the filter section. Negative is meant to be a smoother overdrive, positive is meant to be harder. Not sure if that’s the same as what you’re talking about, I will educate myself later.