The comparison is worth doing properly, because the two ingredients are often specified interchangeably and they are not.
Sweetness perception
Rebaudioside M, the most refined glycoside in the stevia leaf, has a fast onset that closely tracks cane sugar. Its weakness is the tail: at higher concentrations a slight dryness appears, and older Reb-A grades carried a pronounced licorice note. Mogroside V from monk fruit is slower to arrive but adds mid-palate body and a clean finish, which is why it masks protein astringency and botanical bitterness so effectively.
Cost and dosage
Reb-M is typically more expensive per kilogram but delivers 300–350× sucrose sweetness, so cost per sweetness unit is often similar or better in high-volume beverage work. Monk fruit at 200–250× suits applications where dosage is not the constraint and where taste fidelity justifies the difference. A blend lets you buy the onset from stevia and the finish from monk fruit, and brings the total high-intensity load down — which is a sensory win, not just a cost one.
Stability
Both are heat stable well beyond pasteurisation temperatures and pH stable across the range you will encounter in soft drinks, dairy and bakery. Neither participates in Maillard browning, so if you need colour development you need allulose or tagatose in the system as well.
A practical recommendation
Start with a two-point trial: one sample of each ingredient alone at the target dosage, one blended sample at a 70:30 ratio. Blind the panel. In our experience the blended sample wins on overall liking more often than not, and the panel feedback tells you immediately which attribute — onset or finish — your product is actually sensitive to.