In the comics, Stark has a virtual of different armor designs. There are variety of different chest pieces, weaponry, and colors, so no surprise to see him switch from a circular looking reactor to a triangle design I guess. I think you could explain the triangle reactor as either experimental or even just a cover change (maybe its still circular underneath, but the external opening in the armor is triangle shaped to create a different energy beam). Given Starks penchant for fast cars and different designs, it could just be a style choice with that particular suit.