Community-driven tracking posts have converged on a clear answer for the roll-versus-fuse debate. Below Rebirth 4, fusion is mathematically superior because the cost to roll a Mythical Tide aura exceeds the cost of fusing two Abyssal Echo auras by a wide margin.
Above Rebirth 4 the math flips: daily code allowances plus higher per-roll cash pools make raw rolling competitive. This is most visible for players who consistently redeem the BAITAURA and 2500CCU codes.
Extinct Leviathan remains an RNG gate for non-spenders. The 0.05% drop rate combined with a 10x catch-luck modifier means that a single aura may take dozens of days of active rolling for the average player.