the shimanos don't have the spherical washers of the avids, which means you only have to line them up on one axis. unless the frame disc tabs are totally jacked up, that's usually all you need. the spherical washers can sometimes stick and make it more difficult to line up the pads.
the other thing i like about them is the bleed process. with shimanos, you just need their bleed kit, or a bottle of their mineral oil and a typical syringe with a plastic/rubber tube to connect to the bleed nipple. (the bottle of mineral oil can be used in lieu of a syringe) just hook up the supplied hose and squeeze the oil in with the reservoir cap off. it's basically a one step bleed. the avids add about three steps for bleeding, all of which make sense for their system, and require their special syringes with the fittings. the avid bleed process takes about as long as the shimano does, but if you don't have their fittings, you can't do it. at the shop, we've had to bleed about 10 avids for every shimano brake that comes in. my other beef is that you can't pressurize an avid brake while bleeding. with shimanos (and hayes) you can put the res. cap back on and pump fluid in to pressurize and set the pistons, then close the bleeder valve with the hose still connected and you're done. the avids don't allow this, as the bleed fittings thread into the caliper port and lever ports, so you end up losing a tiny bit of fluid ever time you remove the syringe to replace the screw in the bleed port. this can make life difficult on a juicy 5 or 3 if you're trying to get the lever throws even.
my personal juicy 7s have failed twice on me due to a leaky reservoir. SRAM replaced the levers both times, but the second failure was with the brand new lever right out of the box, during installation. i've never had that happen with a shimano brake.
don't get me wrong, i love my juicy 7s, and i've gotten very good at bleeding them, and the process goes very quickly now, i just happen to prefer working on the shimanos. if i was so inclined, i'd get a set of the new XTs, but for now, the avids are working well enough.