We could beat around the bush. We could talk to players who are still on the Mono Blue train. We could talk to players who swear their allegiance to Ephara, God of the Polis. We could cite recent successes with the deck and analyze its place in the metagame.
Or we could just go ask No. 5 Sam Black.
Sam has been synonymous with the deck since he made the Top 8 of Pro Tour Theros with it. It wouldn't be too far off to say he's produced countless articles and videos on the topic, but I'm pretty sure you can count them if you're so inclined. Let's just say he's played it a ton and been a thought-leader on the deck ever since it became fashionable to pair Thassa, God of the Sea and blue mana symbols.
This just in: Master of Waves still good.
Sam has also put in the work testing both variations of the deck—the Mono Blue version and the new version that splashes white for Ephara, God of the Polis and Detention Sphere. Before we get into his thoughts on both, let's take a look at the lists of two grinder-winning decks that chose to go slightly different ways with the archetype.
Let's first tackle the case for original-flavor Blue Devotion, a case Sam was more than happy to make. You know, because he's playing it.
"The mana is much worse in Uw. This deck really wants to curve out. If you play something on turns one, two, and three you're probably in good shape. In Uw, it's hard to do that," Black said of Blue's options.
This also just in: Sam Black still good with Mono Blue Devotion.
Furthermore, Black isn't terribly impressed with Ephara, a point he made clear when he recorded videos of the deck on StarCityGames.com He said that, if it cost 2UU or if all of his lands could be Tundras instead of Islands he might play one just so he can diversify his legendary card drawing engines (Bident of Thassa and Jace, Architect of Thought), but that it just wasn't as good as the other options.
The other big upshot of White, Detention Sphere, only mildly interested Black.
"Detention Sphere is fine, but you really want to tempo people out, and you can't do this at instant speed. You trade your turn for their turn, which isn't really what you want to do," Black said.
Black again reiterated that, even if mana weren't an issue and it cost 2U or 1UU or something similar, he'd still only want to play 2-3 copies.
So why in the world are people splashing white?
"UW is much better against Mono Black," Black said. "Ephara is actually really good there, since they can kill Jace. And Detention Sphere is just gold against Pack Rat and Desecration Demon. It's an attrition game."
Besides the "Detention Sphere is gold" pun, Black's points mostly line up with Brian Kibler's take on the deck as well.
"The white splash makes you more resilient against attrition and Pack Rat, but splashing makes you weaker against green," said Kibler, a guy who's pretty familiar with green cards.
Rapid Hybridization, both Kibler and Black said, is really important against Polukranos. Because the green decks have a better late game, your answers need to be cheaper and give you better options for your mana, especially at instant speed. Rapid Hybridization does that. Detention Sphere doesn't.
"In Buenos Ares, I think the Blue decks were successful because of Rapid," Kibler said. "But I think it's a legitimate question which one is better."
Not for Sam Black, though. Even with all the Tundra's in the world, he knows which variety of Blue Devotion he wants to stick with.