You get both. You apply them in either order, as you control the object/event being modified. The order doesn't matter in this case, because it is addition, but it does for doubling effects. You apply your first replacement effect, and you are left with an object/event that still meets the requirements of the other replacement effect, so you apply it too.
June 19, 2016 11:58 p.m.
Both will apply to the same situation.
Both cards will see Chandra's Ignition dealing damage. Let's say you have Embermaw Hellion apply first. Now the Ignition is dealing X+1 damage where X is the power of the creature you targeted. Then you have Pyromancer's Gauntlet replace that event so now the Ignition is dealing X+3 damage.
The only time you'd have to choose only one to actually take effect is if multiple effects would replace an event with something entirely different, such as having Archmage Ascension and Abundance trying to replace the same draw. If you have one replace it, the event is no longer a card draw and therefore the other can't effect it and does nothing.
June 20, 2016 12:02 a.m.
I just realized that Chandra's Ignition does not deal the damage, only the creature it targets does. Therefore, the Gauntlet does not apply to this situation. If you are not targeting a red creature that is not Embermaw Hellion, the Hellion will not apply its replacement effect, as it does not count itself.
Simply put: you will NOT get the Gauntlet bonus because it is not the correct type of source and you MAY get the Hellion bonus, depending on the source (of the damage).
June 20, 2016 12:10 a.m. Edited.
Well, I messed up on the creature bit but it was the replacement effects I was really looking for. Thank you both.
June 20, 2016 12:20 a.m.
Though its already been discussed why this situation doesn't involve competing replacement effects (and that the order of these particular effects doesn't matter), there's another correction we need to make on who gets to make the decisions about which effect to apply:
If multiple replacement effects would modify how an event affects a player or object, the affected player or controller of the affected object decides which one to apply first, not the "controller of the event" (no one "controls" or "owns" an event). This means in a situation where you're dealing damage to an opponent and multiple replacement effects want to modify the damage event, your opponent would decide the order in which they apply. Again, order doesn't matter in the case of your Gauntlets and Hellion, but it can matter in other situations.