It's not about what spec is fun/better it's about what you need in a given situation.
If you will be in charge of AoE adds tank = Frost
More sustain and better survivability + more health, better aggro and overall more fun if you may, ease the burden on your healers and bring nice buffs to the table = Blood due to self healing potential.
In the end it all breaks down to what you need in the current situation you face, altogether I suggest you try both and see what works best for you, avoiding Unholy tanking because it is not viable at all, terrible unless you planning on tanking dungeons only.

