Hardie Plank vs. Brick
I'm not an expert but I too am considering the use of Hardie Plank and brick in an addition on my house. I would not be as concerned with the insulating value of the veneer material as I would the durability. Neither material has a particularly high R rating. To get the most insulating power, you would probably do a lot better by deciding on the basis of asthetics and durability and utilizing other insulating measures such as sheathing with insulaing foam board and 2x6 wall studs in order to allow for extra thick insulation.
Hope this helps.