Fast answer
Choose Voltfawn if you need a premium carry. Choose Zaplet only when the team already has enough direct damage and wants broader Lightning setup.
Why Voltfawn is usually better
- better as a true main carry
- stronger broad value across modes
- easier to justify as a first major investment
Why Zaplet still matters
- useful on mode boards with crowded enemies
- stronger as a setup piece than as a centerpiece
- good on teams that already know exactly what their carry is
Verdict
Voltfawn is the better account-defining line. Zaplet is the better specialist add-on.