I have been using hydraulic brakes since my Magura pads with motorcycle levers and can tell you honestly I don't remember what v-brakes are like.
Even upgrading though from a pair of Juicy 3's to Formula's makes a huge difference. At 150 lbs both had enough power to lock up the wheel but the ability to modulate the exact right ammount of power to prevent lockups is very easy with a nice set of brakes. Not sure you can get the same level of modulation in a v-brake and probably not at the same level of ease as you can with hydros.
Another thing to consider is the wheel choices you have/don't have depending on your choice of brakes.
What kind of riding are you doing? I don't think the Avid Elixir 5s are very impressive. I have them on my Reign 1. Their stopping power is a disappointment to me.
But for sure they're way better than any mechanical brakes you could get.
If you ride very fast, race, or are really aggressive I'd say don't buy them.
This kids riding a rockhopper, so I assume he's riding tame xc. I would suggest getting some bb7's and set them up properly. Way better than juicy 3's and you will never have to bleed them.