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First off, both are discontinued items, no longer in the uwatec catalog, but I would not be surprised if some dive shops still had some hanging around. They have been replaced by the Galileo Sol.
The Smart Z was the hoseless equivalent to the Smart Com, a great single gas nitrox compatible computer with user adjustable conservativism.
The Smart Tec added multi gas switching capabilities, the computer could plan and manage a dive with 3 different nitrox blends and could read 3 tank transmitters.
The new Galileo can handle 3 gasses, can read 4 transmitters (your 3 plus 1 for a buddy), has a built in digital compass that works great, and as they develop new software for it, it is easily upgradeable at home.
One other difference that prompted me to purchase a Galileo. I have a Smart Tec, and the batteries need to be replaced at the factory (six weeks or so). I had a dive trip coming up, and a battery that needed replacing, to I purchased a Galileo, which has user replacable batteries. Uwatec makes great computers.
Dan