From the basic first time user level....
mechanicals are usually the better choice. Generally mechanical disc brakes, esspecially Avids, are easier to set up and maintain than hydrolic disc brakes. This is mostly due to the fact that most riders understand cable brakes to begin with and, unless they are totaly mechanically inept, can unerstand the basic concept more easily. The learning curve with mech discs is realatively low.
With hydros you're talking about a system that is still realatively new as a bike braking system in most peoples minds. For the entry level user they can be quite daunting if they don't "do their home work" and get as much information about them as possible before jumping into them. The learning curve is quite high for a period of time until they gain a good understanding of the system. They are basically in over their heads for a while. But that doesn't last too long if they are willing to learn. And most people discover that while different, the hydro system isn't that much more complicated than a cable system.
Anyway, as for which system is better, it's a matter of preference. As snaky69 put it, I'd take a set of BB5's or BB7's over a low end, low quality set of hydros any day. I've owned both types of brakes and for ease of maintenance I'd have to go with mechanical discs. They are a breeze (usually) to set up and maintain. For power I'd have to go with hydros. But that has more to do with the design of the calipers and the hydrolic advantage. They do require more maintenance, and a rebuild of the system is more involved. For the level of bike and the brakes that you are looking at, BB5's and Juciy 3's the performance will be about the same. The 3's will require annual bleeding, the BB5's will require annual cable and housing replacement. So between them, let the rest of bike specs make up your mind for you. You can't go wrong with either the BB's or the Jucies. They're both good entry level disc brakes and perform well and are realatively easy to maintain.
Bottom line, both sets you are looking at are Avid, so low quality isn't going to be a problem, either style of brake performs well. For the "disc brake newb" I'd recommend a good set of mechanicals. They perform as well as many hydro brakes for most types of riding, XC, Trail, All Mountain. But if you want to go hydros (as long as they're GOOD hydros), it's not that big of a deal and they work great as well. For you the choice is MAINTENANCE! Do you want drop dead simple cable disc maintenance, or are you willing to put forth a little more effort in learning to maintain the hydros. Your choice.
Good Dirt