Hi guys,
There are pros and cons to both, as always, you have got the most important one as in cost, not only is the belt itself cheaper than a chain but so are the sprockets, you need a hydraulic tensioner on a chain, and you need to lubricate it and have prbably 4 rubbing strips all of which adds up, on top of that there is the noise. perhaps not as obvious as you might think but the chain tends to set up resonances in covers and case that need addressing, for example on some engines there is a bolt holding the cam chain chest tight to stop it 'panting' and making a racket. You tend not to get this any of with belts.
In terms of what to use, belt or chain, you have to weigh up the application, for high revving engines such as bike engines and big power engines the instantaneous stretch in a belt would be prohibitive, lots of cars use chains, such as Saabs, BMW's (all M-engines) etc, I prefer chains as they do stretch over time and once they go further than the tensioner will cope with they rattle a bit and give you some warning to get it fixed, a belt doesnt stretch with time (very much at all) and once fatigued will just break, not good.
So in all chains are dearer but better, belts are cheap and good enough for most 'normal' cars