A pump of standard syrup is about 20 calories and 5 grams of sugar; the sugar-free equivalent is typically 0–5 calories and no sugar, sweetened instead with sucralose or similar. In a grande with four pumps, that is a saving of roughly 60–80 calories and 20 grams of sugar.
What does not change: the milk. A grande latte's roughly 17 grams of sugar come mostly from lactose in the milk itself, so a 'sugar-free latte' is not a zero-sugar drink — it is a no-added-syrup-sugar drink.
Taste is the real trade-off: some people notice a thinner sweetness, especially in hot drinks. Starting with half regular, half sugar-free pumps is a gentler on-ramp than switching outright.