If you do so on a higher difficulty level, you'll gain even more rewards
Sacrificing the creature is part of the cost of casting the emerge spell, so you sacrifice the creature before the spell goes onto the stack and way before the creature with the emerge ability enters the battlefield
If that creature spell has flash, you'll be able to cast it any time you could cast an instant, even on an opponent's turn
Thankfully, the deck comes loaded with a few ways to start pinging your opponents, punishing them for doing everything from tapping lands for mana from Barbflare Gremlin, to having creatures die from Blood Artist
These days, we try to reserve these types of designs for hybrid mana cards
Though, right now while cards and meta decks are relatively limited, you might as well spend away