I do not know much about Hillebergs (prejudiced by their weight) but have been very happy with the Ultamid 2 with Inner Full Mesh over some ~200 hiking days.
I would compare the two as follows given the above caveat:
1. Build quality. I am not gentle with my Ultamid 2 and it has withstood years of abuse. Zippers still work fine; no abrasion or tearing on the DCF. Ultamid 2 and Hilleberg on par here.
2. Waterproof, absorption-resistant, stretch proof, and UV proof. I’d imagine the Hilleberg is on par here.
3. Very light. For a legitimate four season shelter, it doesn’t get much lighter than this for 2 (even 3) people. Ultamid 2 wins here.
4. Versatility. Tarp only on a snowy, windswept ridge. Mesh only in the middle of summer in the desert with scorpions everywhere. Tarp and mesh for some conditions in between these extremes. Pitch high for ventilation and condensation. Pitch low for severe winds. Ultamid 2 wins here.
These were a concern at the time of buying and this is the long-term feedback:
1. Packable Size. The Ultamid 2 is huge when unpacked and it comes in two stuff sacks. But livability was a factor for my son and me. Other two persons were tight on floor width, but this is very roomy. DCF packs very small and I figured out how to pack both the tarp and the mesh into one provided stuff sack instead of the two (when folding lengthwise, do one extra fold and roll very tightly while someone holds it taut). Packed thus it fits into my tiny 38L MLD Burn with room for all my other gear inside the main compartment. Size hasn’t been a problem.
2. Usable Space. With a one pole pyramid design, you lose some space at the edges. This has turned out to be a blessing. I wedge gear into the edges, which increases headroom, and have everything at hand when I need it. Rodents and mini bears can be a problem in most places, but not normally when gear is indoors.
3. Two Pole Strength. I use the Black Diamond FLZ Carbon poles lashed together and was concerned that they would not hold up to high winds. Since then we’ve had +100 km/h (60 mph) winds without event. In milder conditions I have the option of extending the poles and moving them right against the wall. Even if poles break, I could tie the peak to a tree or use a stick, an advantage over a freestanding shelter.
4. Cooking in Rain. I was concerned that in rain and wind cooking would be difficult. Not a problem. Pitch with the door on the leeward side, pull the floor back to create a vestibule area, place stove on ground, open door halfway and cook. Rain has never been a problem set up this way, something we’ve tried with four full grown men sitting inside.
A friend recently asked for a shelter recommendation if he were to buy just one tent for everything. I told him the Stratospire 2 in Silnylon for the price, the Durston X-Mid if floor width is not a concern, and the Ultamid 2 for everything else.
