The short answer
Both, depending on how often you run them. A SPIFF is a tactical lever for a specific short window: a product launch, a stalled segment, a quarter end push. Run one a quarter with a clear metric, live visibility of standings, and payment within days, and you get real lift. Run them constantly and reps start sandbagging deals into the next one, which is exactly the behaviour you were trying to avoid. Two design rules matter more than the amount: never winner takes all, because the other 90 percent check out immediately, and budget it as a percentage of the expected revenue lift so it does not quietly become a permanent line item.