I tend to think Iginla can be overrated, as he had some up and down years in his prime and his power forward/all around player reputation followed him into the years where he had become much more of a pure shooter, but at the same time Stamkos has had down years of his own (mostly injury related), never brought the all around game of Iginla's peak, and has had far more time with high end linemates in his career.
Regular season prime is actually fairly close, even with Stamkos's age, as he entered his prime much younger, and Stamkos should probably pass him there with a reasonable finish to his career to overcome Iginla's current longevity edge. However, until Stamkos can show he can bring it in the playoffs and not be continuously disappointing, it would be hard to consider him the better player. Iginla had some disappoints as well, but was far above anything Stamkos has done in the finals run in '04