I looked at the Unna and the Soulo for my first Hilleberg. For winter use, I decided that the Soulo was more capable with its third pole, both at withstanding strong winds in the mountains, and at handling snow. Mine handled two feet of wet cement-like snow in an overnight storm with ease. You will still need to deploy the guylines in conditions like that, so carrying a mixture of pegs capable of penetrating frozen ground is a must in winter camping.
What do you give up opting for the Soulo? The inner isn’t as large. I’m 5’9″ and I was perfectly happy. I’ve read folks who are 6’2″ and were also comfortable in it, so it’s a matter of what you’re willing to deal with. I was able to put a large / wide Exped Downmat in there and still have space left over. It weighs 7oz more than the Unna.
What do you gain, apart from what I mentioned above? A real vestibule for one. One you have space to cook in. An inverted T zip door on the inner, which can be opened up fully, left half open, closed with 1/2 mesh, or closed with full mesh. Very flexible. A decent roof vent. Not as effective as, say, the 2P Jannu or Allak, but combined with the vestibule, I would say it gives better air circulation than the Unna. And lastly, incredible stability. The Soulo pitched taut as a drum, without stressing the outer tent zip. It really feels rock solid.
Another thought, for condensation management and ventilation, you should expect to have the inner tent door partially open. And in still conditions, open the outer tent door somewhat too. (When there’s good airflow, the roof vent should be enough). I felt that the Unna’s inner tent door shape and zipper setup made it nigh on impossible to roll 1/4 – 1/3 of it back and secure it to allow more airflow to the inner. Perhaps people who own Unnas can confirm or refute that.
Best of luck in your decision.