This is important because cards like Desperate Ritual, Rite of Flame, and Manamorphose suddenly are playable for free! You can create infinite mana with the Ritual and Rite of Flame, since they remain on the stack to copy. With that mana, you just cast Banefire, burning them infinitely for the win. In fact, Rite of Flame is so powerful in this combo that it’s banned in Modern for this very reason! It’s still playable in Modern with the other two cards, but the deck now only mostly sees play in Legacy for that reason. Talk about a combo enabler!
In Commander, he’s a tad more versatile, and that first ability is more relevant to filter dead cards out of the hand. But Izzet Guildmage is still the far better card here. He’s still Constructed-playable, but currently, he’s not nearly the combo enabler of his senior associate and simply a useful creature that combos well with Manamorphose (which creates two of any colored mana). Having the Blue/Red combination there hurts it with any of the Red mana producing spells besides that one. Sorry, Nivix Guildmage, but you don’t stand a chance in this match-up.
Winner: Izzet Guildmage, for making Rite of Flame banned in Modern.