Seconded.
I would think that even 3v3, if teamwork was heavy, that ice TB would own compared to fire. Definitely once you go 4v4 or more, I think the AOE of his ice path would prove to be better. Slowing attack/move speed, freezing in place, interrupting skills, increasing their cooldown, shattering armor... Probably the second best support hero in the game. (It's hard to beat Sedna's healing.)
The dynamics just change with more people, and I think it is reasonable to expect that you would have very different results playing different ways in those situations.
I don't tend to take Oak's shield. I favor his stats instead. I bet though in 5v5, I'd take his shield every time. 4-5 seconds of invuln to 5 people wailing on you is much much different than 4-5 secs of invuln to two people wailing on you.