For all the reasons listed, I vote Fire, and also because I still the Fire/Grass type that will eventually come!
However, the Fire types have few useful type combinations to make it a bulky pokemon (Steel being the only one, I think, which is already taken by Heatran).
The Fire/Grass combination is definitely not a defensive Pokemon (directed towards whomever pointed it out), but more of an offensive Pokemon with decent defenses that allow it to get around.
It's weakness/resistance spread is far from "impressive," as the only key resistances it obtains are electric and grass. Being neutral to a lot of types means that it will have a hard time switching in without the correct defenses. That further proves it is better off as an offensive Pokemon.
Not to mention, Fire/Grass is an interesting attack combination. While it does not cover everything, it has enough punch (given the right attack or special attack, but most likely special attack) to leave dents into most Pokes. Hidden Power allows you to select between two of its main walls: dragons or Heatran. In this way, it can somewhat be compared to Yanmega in that Yanmega also has to choose between two when it runs Hidden Power.
In conclusion, I fully support the creation of a Fire type, especially with Grass as its secondary typing.
EDIT: P.S. Fire/Dragon sucks. It has a high potential to become a fanboy-magnet Pokemon!