I tend to mix both, as if you go pure fire all you are is burst'ish damage. Throw a fireball every CD and thats about it, and CoF every creep wave. You offer less help than a half-decent reg sniper with absolutely nothing else to offer (like mines and slowing attacks).
Going pure frost you're not very useful, but more useful IMO than pure fire. Chances are you survive a lot better as pure frost than fire.
Mixing both I tend to max fireball/deep freeze w/ synergy, frost nova, rain of fire, and an aura (usually depending on team makeups). Switching constantly to maintain the regen to stay on my lane for a long time. Its more of a mid-late game build but you offer a lot more utility AND damage to your team by getting both complete with burst damage, aoe farming, a fairly big aoe stun, and the CD interrupt.
F/F mix TB imo is very good early game to farm and help with cit upgrades, and mid game he starts to shine when he can rain/fireball somebody for a hefty amount of damage out of nowhere, and still be completely out of harms way. At the same time because he is mostly just spell damage he can be very useless late game as gear doesnt scale with spells as well (exception maybe -cd timers) compared to an UB or EB
