Belts tend to fail abruptly and catastrophically, but generally only if they're neglected and used well beyond their design life. Chains fail more insidiously, because they get longer over time. The pins eat away at the plates, increasing the spacing between the rollers. This isn't immediately noticeable, because the timing sprockets will wear to match the altered dimensions of the chain. Worn chains cause all sorts of subtle timing problems and the lifespan of a chain is much less predictable, because it's highly dependent on lubrication; missed oil changes or poor oil quality will drastically accelerate chain wear.
From an enthusiast's perspective, a chain is probably preferable, because it tends to give you some warning before it fails - you might hear a rattle at idle, you might have problems with starting or misfires, you might get a check engine light and an EBD-II code from the timing sensors. That's a much less scary prospect than the potential for a sudden and catastrophic failure of a timing belt.
From a manufacturer or dealership's perspective, belts have some clear advantages. Unless there was a manufacturing defect in the belt, they'll almost never cause trouble if replaced according to schedule. That has obvious liability implications - it's not their fault if the customer ignores the specified replacement interval, but it potentially is their fault if a chain wears faster than expected. BMW, VAG and Jaguar Land Rover have all recently settled class action lawsuits related to the premature failure of timing chains.
I'm more of a hobby bicycle mechanic. Isn't there some similar tool for the meassuring of the chain as you get for bicycles? Just push it down and you directly see if it's time to change the chain. If not done early enough you have to change all the sprockets too because of wear. If done in time, you might not have to change all the sprockets.
You can measure it with an ordinary caliper, but the problem is getting access. The timing chain needs to be continuously supplied with oil, so it is in rather than on the engine and can rarely be accessed without pulling the engine from the car. It takes several hours just to access the chain for inspection, so it's not something you'd periodically check - if you suspect a timing chain problem, you might as well replace it.
From an enthusiast's perspective, a chain is probably preferable, because it tends to give you some warning before it fails - you might hear a rattle at idle, you might have problems with starting or misfires, you might get a check engine light and an EBD-II code from the timing sensors. That's a much less scary prospect than the potential for a sudden and catastrophic failure of a timing belt.
From a manufacturer or dealership's perspective, belts have some clear advantages. Unless there was a manufacturing defect in the belt, they'll almost never cause trouble if replaced according to schedule. That has obvious liability implications - it's not their fault if the customer ignores the specified replacement interval, but it potentially is their fault if a chain wears faster than expected. BMW, VAG and Jaguar Land Rover have all recently settled class action lawsuits related to the premature failure of timing chains.