I have both the A4 and Pro2, and no experience with the Pro12.
In terms of sequencer features, the A4 (and AK) is definitely the most powerful of the two. While the Pro2 sequencer is easier to use, it’s simply not as strong and flexible as the one offered by Elektron.
In terms of all-in-box-ness, again the A4/AK gets my vote. It’s basically 4-voice multi-trimbral with the additional option to go poly. It has a solid delay as well as a reverb. You could do whole compositions indeed on the A4/AK, except maybe for EQ (and some compression).
The Pro2 otoh is single-timbral, single voice, with the option to go paraphonic, and I have to say this works quite well. No way though to easily record separate parts since only one sequencer track in the Pro2 will do notes atm. Pro2 has 4 delays, but with shorter max delay time, and no reverb.
In terms of user interface, Pro2 all the way. Done.
In terms of synth engine, that’s a tough one and comes down to personal preferences. The sound of both synths couldn’t be more apart. The A4/AK is an analog synth (under digital control) and sounds just great imo, with all the traits you’d expect from a juicy and warm analog. As mentioned by pulsn, it reminds me of the Roland SH5/SH7 synths, which is good, and I understand even some of the SH5/SH7 sync tricks are in.
The Pro2 sound is completely different. It has a digital core and that’s immediately obvious when you play it. The sound is more static and stable. It’s a good digital core though, and a single OSC’s saw tooth for example gives me a good body of sound in the lower register. The addition of the two new analog filters on the Pro2 was a great choice, and can give the sound analog warmth and saturation, if you want that. Whatever you do, there’s always this nice fat bottom end.
The A4/AK sound palette is limited to relatively warm and lively analog sounds, but it does analog very well imo, with rich set of features that I haven’t seen before in analogs.
The Pro2 sound palette is massive. I think this is the most expressive synth I’ve ever owned. It can do pretty nice warm analog style sounds complete with analog filter overdrive and the works, very pure sounding sine waves, and all the digital power you’d expect from a 4-voice that can FM/AM anything to anything. Although it’s digital, this synth has class all over.