Is your architect recommending board and bat for the new gables as a cost-saving method? Or does he just like the look?
If it were me, and I had horizontal siding on the original house, but I would be bothered by the vertical lines of board and bat. However, have you considered shingles... wood, not asphalt. If might make it look a little more like a New England salt box house, and the rows of shingles would continue the horizontal look of the original siding. Just a thought.