If you need physical outs for each track then A4 doesn’t have that, only AK.
You can program the joystick functionality into something else, but it’d be a pain in the ass.
Overbridge is going to make them basically the same, you’ll be able to multitrack on the A4.
If you already have a really nice keyboard- go for A4, no need to spend the money.
I will say, the AK keyboard is super nice and responsive and I often use it as a midi keyboard for soft synths. The ability to program CC on the knobs is something I don’t think A4 does and it feels nice to use the knobs on the Elektrons, I really like their feel.