I adore both for both GY shenanigans sake and for supporting spells decks, but they've been comparatively unpopular to other two drops and have been at the cusp of my cube for the last year. I'm committed to keeping them in over generic aggro beaters, but it's hard to justify two dudes with 1 base power at such a competitive MV in such an aggressive color, no matter the other context.
Personally, I only have 30 instants or sorceries at 1 MV in my list of 720 cards -- that's just 4% of the cube. As a result, it's really tough to use
Dreadhorde Arcanist without any additional ways to buff him. There are plenty, sure, but unless you can make it a sure thing, his utility is fairly limited.
Magmatic Channeler, on the other hand, is an engine all in herself, a 'Goyf that can feed herself. But still, not a high pick for Rx aggressive decks unless you're running 8-10 spells, which you'd be lucky to get with the amount of competition for burn spells. All the same, if I could only keep one, I'd keep her for her increased flexibility, even if she doesn't have the classic border treatment or as high of a ceiling.
@Chris Taylor, looking at your list, I imagine both would be incredibly good and fit in quite well with you red twos. Loving the custom cards, but I do gotta ask -- why run two
Heartfire Immolators? I get the double Pyromancers, it's a great glue card and really contributes to your spells-matter package. But both of these we're discussing do as well, and the redundancy in Immolators is less necessary thanks to your prowess-having customs.