I think it depends on your style of wheeling. I've been on enough runs with guy's running Iroks/Toyo's, and from what I've seen:
Iroks excel in mud, lose gravel, and surprisingly they kick arse on lose & hard packed snow.
Toyo's, they're great in snow, lose gravel and rocks, not so great in mud. But if you try and avoid mud as much as possible, you can't go wrong with toyo's.
Fiend was able to followed my around, climbing steep steep hills with over a foot of snow. He was Open/Open, I was locked and chained, mind you I was breaking trail. But the fact that he was able to walk up on those steep hills, was amazing.