I've had both (Kifaru 4-man, SO 6 & 8 man, SO LBO base/tarp/LBO base).
My preference is for the SO, because imho they are better built, offer much better value for money and the owners don't get hung up defending the indefensible (in Kifaru's case that relates to their stance on the strength or otherwise of their guy-out points, and having a proper zip cover). SO, for example, changed their tent build (from asymmetric panels to symmetrical) because they recognised it improved the tent strength.
A discussion on the Kifaru V Seekoutside tipis can be found here https://www.hillpeoplegear.com/Foru...id/9567/scope/posts/threadpage/1/Default.aspx
Yes, I use/have used a woodstove in all my Kifaru and Seekoutside tents. Ventilation is not a problem - the stovepipe itself goes out through the tipi/tent at the top, SO tents are provided with two high-level ventilation systems, and additional ventilation can be provided by raising the door zip a few inches to increase throughflow if needed.
Stoves used to date are all lightweight "flat-pack" stoves. Kifaru medium (stainless steel), Seekoutside Large (titanium and Hillpeoplegear Shepherd stove (SS). All work well - produce plenty of heat but will not burn for more than a couple of hours per wood-load, so will not burn throughout the night without reloading. On the plus side, the stoves, including roll-up pipes, are very light. THe SO large, for example, with 9ft stovepipe, is less than 2kg.
Re your choice of tents, I believe the quality of the Seekoutside tents is first-class. As far as size is concerned, its all down to how light you want/need to go! Unless weight is ultra-critical I'd accept the additional kg and go for the 6-man. My own 8-man SO tipi, including large SO ti stove and stovepipe, weighs less than 11lbs/5kg.
6-man tipi at -20C? All of the ones I've mentioned (although Kifaru medium would be a bit marginal) There is also another "fold-flat" stove manufacturer - Titanium Goat http://www.titaniumgoat.com/stove-WiFi.html . Their Large Wifi ti stove would do well as well.
As to which is best......
I don't have much to add, but I used a Kifaru tipi for about 10 days down to -15degC in Alberta Canada, January 2012 (the -25deg cold snap ended the day we arrived ) Ours was the 6-man model and it was a tight squeeze for three plus gear. So tight that one of our number de-camped and went and set up in a hammock for half the time! We had one of the fold flat stoves, medium I think, whatever it was, it wasn't great at raising the temperature. The gap around the stove pipe exiting the heat resistant jack panel allowed hot air to escape before the lower part of the shelter ever really warmed. When sitting on our mats, there was a frost line at chest height. I guess though that this is all relative, it was a lot warmer inside than outside. The impermeable material the tipi is made of meant that we got lots of frost on the inside, and the stove never melted anything lower than 2 feet above ground. I am very glad that we had other cooking arrangements and did not have to use the stove for that too. We were able to borrow a Fourdog stove for the second half of the trip and that worked a lot better. It wasn't a fold flat, so had much better draft control, and it was able to raise the temperature a good deal higher. Actually, the first time we used it, it went a little too high. The Kifaru, due to poor air control, only really had two settings, flat out glowing red, and going out, so we always kept it stoked. Doing the same with the Fourdog meant that the upper part of the tipi, shoulder and head height when standing, got up to over 50degC! Didn't hurt the tent, but did rather shock us.