Doug is far more experienced than I am and I've learned a ton from his reviews and articles and PMs (thanks Doug!). And he is absolutely 100% right about ease of tunnel setup in high wind and about the extra space afforded by vertical walls (which is why they are often used for expeditions, where people may spend a lot of time in their tents with their gear).
The Kaitum is a very roomy tent for the weight (and I know Doug loves his). I checked it out very closely when evaluating tents. It also requires more space to pitch.
And the Allak does have clips beyond the first sleeve section, and when you erect the shelter, you clip onto the poles moving in a circular pattern around the tent, so that one side doesn't come way up before the other and make things more difficult in wind.
That said, the same things I mentioned above apply - once set up, a tent like the Allak or Jannu is going to shed wind better, deflect less, handle snow better (so you don't have to worry about constantly shaking it off), make less noise, etc. These tents will also fit onto tighter spaces than long tunnel tents.
The double vestibule on the Allak is plenty space for 2 people to each store their gear and get in and out of the tent easily, including winter gear. It's a palace for 1 person.
What it comes down to is how you are going to use the tent and what you want out of it. 2 doors or 1? How much space? Do you plan to spend a lot of time in it? Do you plan to carry it a lot, or will it travel in your kayak or on your pulk?
The Nallo 2 is a little lighter - it's also shorter, has less space and has only one door and vestibule - a deal breaker for my wife and me. For others one door may be fine. Again 2 doors for 2 people = better sleep. Also the Nallo's less space may feel like more because of tunnel walls. I've never felt cramped in the Allak sitting up or lying down and I'm 6 ft tall and my wife is 5 ft 6.
Before I pulled the trigger on the Allak I spoke with local backpackers who winter backpack and do mountaineering and I posted a lot of questions to this site (you can look up the long thread). Many of the winter backpackers I spoke with, including another BPL staffer in my area, strongly advocated for 2 doors and a crossed-pole dome design. I also managed to get a great deal.
Note these 2 long very windy kayak trips the Allak was chosen for - you may want to write the kayakers and ask why they chose Allaks and how they performed:
“Circumnavigating Newfoundland" On the heels of his fast record-breaking circumnavigation of Iceland in 2007 by sea kayak, Greg Stamer will be embarking on a fast solo circumnavigation of Newfoundland, “the Rock”, Canada’s easternmost province in June, 2008. This adventure is more than 1,700 miles around one of the most windswept locations in North America. For pitching in exposed coastal areas on both sand and bare rock, Greg will trust a freestanding Allak to keep him comfortable and safe. Read more about Greg’s journey at www.gregstamer.com"
"Freya Hoffmeister". In 2007, German paddler Freya Hoffmeister kayaked around Iceland in a record 33 days. As a follow up, she set out to became the first woman to circumnavigate New Zealand’s South Island by sea kayak. 70 days later, she had succeeded, setting a new solo record in the process. Her latest successful adventure in 2009, The Race Around Australia Expedition, took her roughly a year to complete. The 9400 mile/15000 km circumnavigation of Australia has only been done once before in the last 30 years, and is considered one of, if not the, most challenging sea kayaking routes in the world. She got her well deserved rest each night in an Allak. Read more about it at www.qajaqunderground.com
As for the Scarp 2, Henry makes quality shelters and his customer service is great - just ask Scarp 2 users online and offline by PM how they find the shelter in strong wind. I'm sure there are plenty of users out there.
Best of luck. I'll be happy with whatever you buy. I put in the time to offer feedback because many kind people on this site helped me so much with my choice and so many other gear choices and techniques.