Classic+ or Legacy?
Every character picks its skill mode once, for good. Choose a class to compare each of its skills in both modes: damage, cooldowns, protection while casting, cancels, projectiles, buffs and transformations. Only what differs is shown.
Classic+
The classic skill trees, kept up to date: it keeps getting new skills and quality-of-life changes, and it can use Talents.
Legacy
The NextGen skill trees as they were before the classic restore. No Talents, no new skills or quality-of-life changes, and still subject to damage balancing.