Gauges are a tricky subject. Many people have there own favorite brands, mine being ISSPRO, so I would be a little biased.
As far as price is concerned, $100 a gauge will buy you pretty good gauges, but not the bets. I like to think that ISSPRO is the best, obviously cause I'm biased
. But I also think that other name brand gauges such as Auto-meter are good gauges as well.
Mechanical gauges are the old school technology gauges. For instance a oil pressure gauge will have a line comign from the engine with engine oil to measure the oil pressure. They are generally cheaper, but if something goes wrong, if you have oil pressure gauges and what not, you might get oil into your cab.
Electrical gauges are the new technology gauges. They will have senders that read pressure, temp, ect...then wiring coming out of the sender that goes to the gauges. They are generally more expensive, and there is more wiring involved.