While I was wandering through Wal-Mart, trying to decide whether or not Essie Mojito Madness was too similar to Orly Mint Mojito, I scoured the internet on my smartphone trying to find some comparison swatches. I bought it anyway, despite the lack of comparisons; not sure the outcome would have been different had I found those swatches...
From these pictures, you can tell that these two are not exact dupes of each other. Mojito Madness has a much more saturated colour, while Mint Mojito is slightly paler and dustier.
The flash pictures show the difference a little better:
So, do you need both? Probably not. To the average person, these colours probably look exactly the same. From a distance, it's hard to tell the difference, but they are definitely not the same. You will probably only want both if you LOVE THIS GREEN, like me. If you were to only get one, I would say the Essie is the best choice, based on formula.
This picture is after a single coat of polish. Placed right, the Essie comes close to being a one-coater. The above swatches were 2 coats of the Essie, 3 of the Orly. The Essie reaches opacity much quicker, and is also probably easier to find.
I got Mojito Madness at Wal-Mart for $8.99. Mint Mojito was $4.99 (+ $5 shipping) from an e-Bay retailer.