If you cast a spell for another cost "rather than pay" its mana cost, you can't choose to cast it for any other alternative cost
The Ring's emblem of temptation relies on players having good creatures that can attack effectively without being killed by blockers too easily, so the effect may be better used by creature-heavy decks that are already adept at pushing damage, or even control decks that can attack with a single, evasive creature to close out the game
For six mana you get an enchantment that has you swap control of Conjured Currency with a permanent that you neither own nor control
Faerie Vandal can grow into a massive flier as we draw extra cards each turn, while Evangel of Synthesis is usually a menacing 3/3 attacker for two that also triggers all of our "draw a second card" creatures when we play it, making it a decent threat and a solid support card
It costs mana to activate, but its also a 1-drop and much less restrictive than Knight of the Reliquary, so it has some advantages that the Selesnya card doesnt have