I have both. The Avid mechanicals are great for running any lever you want, they are rock solid and reliable, super adjustable (getting the rotor centered is a brain-dead operation), they *can* offer great lever feel if you set them up with good levers, cables and housing, super-super powerful and quiet in most conditions, and they are cheap. They are however bulky, heavy and pretty ugly.
The Juicys have that coveted "hydro feel", are also quite adjustable, look the dogs danglies, are also super powerful (slightly more-so than the mechs with the same rotor size), and offer superb lever feel and modulation. They cost at least 30% more than the mechs do (when you include levers and cables, etc), are more complicated and may have more things wich could go wrong or strip/sieze with all those little seals, dials and such (they've held up great so far- 8 months), and they squeal more than the mechs so far in my experience, though a lot of that can be cured by using the old-style round rotors, EBC Gold or the new Avid B4 pads. The Juicy's seem to be more fade-resistant too but this may be due to the wavy rotors since both brakes use the same pads. Bleeding is very easy (I think) and the maintenance has been reasonably low (occasional recentering using the CPS mount, sticky pistons once).
The only thing which I like less on the Juicys than the mechs is the fact that the pad/rotor clearance is so tiny. When the pad retracts, it only moves about 0.5mm from the rotor whereas the mech's pad can move back up to over 1mm. That makes a big difference in how precise you have to true your rotors and how often you hear a squeak from the rotor rubbing the pad. This is not a big deal on the 160mm rotors, but I think would turn into something annoying on the 185s and up. I would ask for more pad retraction on the Juicy if I could. The greater leverage of the hydros is why they are more powerful, I guess.
For high-zoot looks, cutting edge perfromance, and buttery feel, go Juicy.
To save a buck, get proven reliability and rock-solid performance, get the mechs.