The second ability is good, but a bit high on the cost (3WU). This ability allows you to detain target non-land permanent an opponent controls. This means that if it’s a creature, it can’t block or use its activated abilities, even mana abilities. If it’s an enchantment, planeswalker, or artifact, it simply can’t be used unless its effect is not activated, such as an enter-the-battlefield trigger. But 5 mana is a huge investment for that sort of effect. Then again, because it’s a repeatable and not a tap ability, it makes sense that the cost would be intentionally fairly high.
Overall, the main reason to use New Prahv Guildmage is to give your bigger creatures flying. The detain ability will only be used if you get mana flooded and need something to sink it into. It could save you here and there, but it’s not an ability to rely on.
So which of these is actually better in the long run? The fact that the Azorius Guildmage can react to an activated ability rather than simply detain something may be a bit more useful. Then again, tapping a creature rather than giving your own creature flying may not be as good.
From a Limited perspective, New Prahv may be better simply for the flying ability and the ability to detain whatever it wants (even at that steep of a cost). Azorius Guildmage has more utility in a format like Commander where tapping the right creature and countering the right ability is more important.
The battle here looks to be a wash. They’re both very good, but the hybrid mana cost of Azorius Guildmage and the ability to counter an ability rather than detain something might allow it to win this round. But it’s very close.
Winner: Azorius Guildmage, by a hair.
Let me know if you agree or disagree and why!