Pick the spaced-repetition scheduler. SM-2 is the original SuperMemo
algorithm we've shipped since v1. FSRS-5 is a more accurate two-component
memory model with better retention/effort. Either is safe — switching
only affects the schedule of future reviews.
What actually changes when I switch?
SM-2 doubles intervals on each correct answer, halves them on a
wrong one. Simple, well-tested, but assumes every card grows the same way.
FSRS-5 tracks two numbers per card — stability (how long
you can go before forgetting) and difficulty (how shaky this specific
card is for you). It spaces stable cards more aggressively and shaky ones more
conservatively, so day-to-day review counts are typically lower for items
you've been getting right.
Switching is reversible. Existing review history isn't lost; the next review on
each card initialises FSRS state from your most recent rating.