I am currently asking myself the same question. GU installs variant 3 with concentric air supply from Erlus. In this variant, Erlus is probably really the best, since the chimney there has larger external dimensions compared to the competitors and thus more space for fresh air supply.
I have currently spoken with four stove builders. Two prefer the Schiedel variant because they say variant 3 is no good. But that is probably a general statement, as they have never tried it and are generally opposed to this type of chimney. One stove builder said he only does it with air supply from the basement. Everything else is rubbish. But since my office is below, that won’t work. The last one said all three variants are fine for him. He also builds many stoves with chimneys of variant 3 and has never had problems. He also gave me some customer numbers, whom I then called and who confirmed the experience to me.
It may be that variant 2 works a little better in extreme weather conditions. However, after some research, I also found a few disadvantages:
[*]higher space consumption than variant 3
[*]somewhat more expensive than variant 3
[*]chimney outlet and air intake should always be in the same wind pressure area. With the solution of chimney + separate air duct, this is not always guaranteed, as the air duct can lie in the wind shadow of the chimney draft.
[*]more prone to mold formation (although with Schiedel this is probably solved through better insulation)
Well, in the end, I am wavering between variant 2 and 3. I will probably go with the variant suggested by my preferred stove builder.