Anyone who had all 3 at hand?
I have all three.
My focus originally was on 60fps, out of these 3 only the Plus is able to do that, right?
A139 can also do 60fps on the front when using 2 channels only.
A129 Pro can also do 60fps at 2K resolution, but you miss out on the 4K resolution.
Is that even an advantage in day to day situations over the 4k Pro?
Only for the race track and movie making.
30fps is perfectly adequate for accident footage, and for movie making you can use an editor to interpolate the video up to 60fps to get viewing smoothness, so 60fps is really only needed for racing.
Or would you prefer the higher resolution?
Definitely, although the higher resolution sensor is less sensitive so suffers more from motion blur, so the A129 Plus and A139 are preferable in low light and darkness. It is not a huge difference but if you often drive in the dark then it is enough to make the 2K sensor the better option.
And for a pure 2ch system, the A139 wouldn't even have any realistic advantages over the other two?
The A129 Plus has a lower spec. processor than the other two which affects a few things, most obviously relatively poor wifi performance.
The A139 was designed as the premium model, so has various things that are just a little better, such as the option for an external microphone so that it doesn't have to pick up windscreen noise instead of voice. Overall it is the best unless you want the 4K resolution.
The A139 doesn't have a screen, so it can be properly hidden behind the rear view mirror. It is the only one with a voice, so press the mute button and it will tell you that the audio is now muted, whereas the others will beep, but without taking your eyes off the road and focusing closely on the tiny microphone icon on the little LCD screen, you don't know if mute is on or off! Being the premium model, it also has a good quality speaker for the voice, it is the best sounding dashcam you can buy! It also uses coax cable for the rear camera which is much thinner than the USB cable the others use, many people prefer it for ease/neatness of installation but it also provides reliability with a much better power supply to the rear camera, just little things, but they all add up.
The A139 is just better, and compared to the A129 Pro is the newer and better supported. Currently it is the only one with a geofencing option, although that is still in beta release firmware.
Personally, if price wasn't an issue, I would definitely choose the A139, however they do all work and all record dashcam footage, so if price is an issue don't worry about choosing the cheapest!
The Plus probably does best in summer heat if you want to use parking mode at midday in full sunshine with all the windows closed tight! The others may shut down in such conditions, and wait until it is cooler again.