I have an Atko and a Copper Spur 2.
Hilleberg tents are bombproof. I have had my Nallo GT3 in 60-80 mph winds last year in AK and it never budged. I would trust my life in it in bad weather.
Specifically for the Atko...in snow over 5 or so inches, it accumulates on the end and may slightly lean in...pending on the density/weight of the snow.
I had my Atko on a backpack Nov deer hunt in CO in 2010 and we took around 5-6" of snow one night. The ends would sag in and lean in on my head.
Granted I am 6'8" and take up the entire length of this tent.
The Copper Spur is a 3 season tent as you know...I like mine cuz it gives me a bit more room in the tent and I can sit up in it...unlike the Atko--consider my height here as the avg guy may be able to sit up in the Atko...but not comfortably.
Each tent has its place...4 season tents are best in super cold temps and potential snows. My Copper Spur gets the nod during archery season. Also consider your sleeping bag...a 4 season tent has the ability to be much warmer as you can close em' up. I use a Western Mountaineering Badger with a bit of overfill...its super warm and I have never been cold in it. So that can come into play as well. I have also used the Copper Spur during Oct rifle elk hunts where we took a bit of snow and it worked great.
Both are great.