I would agree that both are good and depend on the other two characters you choose to run with said character. In my opinion, the early lightning ninja trump the early fire ninja, if you plan to run the stat boosting passive. These include ninja like Sasuke, Kakashi, Hinata and Guy, all of whom being free/easy to get. Fire ninja only get two super useful early ninja, Karin and Kurenai. The issue here is neither is massive burst and Karin can't really utilize Iruka passive.
However, if you look at the alternative abilities, fire mains have one that easily makes them irritating right away, Genjutsu Mirror. This was the bane of my existence early game and still hinders quite a few people even at level 70. Fire main already does a lot of damage and with Iruka it is even more. The genjutsu mirrors, on top of Iruka shield, help to make sure that all your dps can combo and stay alive. In my opinion this gives fire a nice bump because you can stack more damage into your team without fear of being nuked or paralysed too fast.