I would hope that direct hits from a grenadier and direct hits from a flame infantry would have pretty similar effectiveness.
It seems like a handful of flame-inf or grenadiers, if able to survive the shots of a single tank, should be able to take the tank out.
Though one shouldn't expect either to be very effective alone against a tank, just do some damage.
Don't give this opinion too much credence though, I haven't paid a great deal of attention to the damage dynamics of free characters.
And generally, when your down to free characters without vehicles, there's not as much hope for victory in the battle.