As a somewhat experienced Frost Bearer, I'll give you some pros and cons that might explain it.
Fire TBs will move faster than Frost TBs due to their auras, HOWEVER, Frost Presence (I THINK thats the name of the aura,) Slows EVERYONE around you down. This isn't gonna stop that Rook from catching you by itself, but with the help of speed items (Boots of Speed, Swift Anklet etc.) You'll be able to outrun most Demigods.
Fire's spells only have one fuction: Blowing stuff up, Frost's spells have two. Damage or control.
Hail of Ice slows attack speed of Demigods by a big amount.
Frost Nova (After the initial stun) Cuts movement speed by roughly half.
Biting Chill (This is acquired by stacking Hail to max level) Even Stops Mana and Health Regen.
Deep Freeze has two main effects you have to decide on, Shatter Combo and Interupts. Shatter is where most of Frost's Damage comes from, the other source is hail.
If you use Deep Freeze to interupt, odds are you will not shatter. If you use Deep Freeze for Shatter, and the target isn't dead, you are wide open. This situation dumps all sources of control out of your hands and into theirs.
If you play with a survivalist mind set, I find you do alot better as a Frost Bearer. OH! One more thing.
Heart of Life. Get it whenever you can after grabbing Boots of Speed and the Pleanor Crown.