Shimanos are great! I personally love Hope brakes, partly due to the feel (the old mini lever fits my hand perfectly), partly due to the lovely machining, and partly due to the performance. However, as I can't always afford those my next brakes are a set of the older Saints. I've borrowed bikes with Shimano brakes, I have a number of riding buddies that ride Shimanos, and I've read many great reviews of Shimano brakes. I've never heard anything saying that they are prone to overheating, and certainly not any more than other brakes.
I do have a pair of Juicy 5s as well, and they are alright, but they are by no means a great brake. They sure felt better than the old Hayes Mags I had kicking around, but in comparison to the Hopes and Shimanos, they aren't very special. I haven't had too many issues with mine, just minor stuff, but I'm not very keen on Avids anymore.
I would say Hope brakes, if they feel good for you, and failing that I would very strongly recommend the Shimanos. For AM just about any of them should be fine I would think, LX, Hone, new SLX, XT, XTR, Saint.... the new Saint might be chunkier than you want though, I'm not sure of its weight.