corks CAN contain "cork taint." might be another term, but regardless it WAS a bigger problem than it is now. many cork producers have scaled back their chemicals and work hard to avoid having a bad cork. many of them use lots of high tech equipment including an scanning machine to check for defects. they are sorted by hand too.
screw caps are good for fruit and low age type wines. most wine producers that use screw caps or synthetic corks, bulk age the wine some to age it first and is pretty much ready for consumption when it hits the shelf. a large majority of people buy their wine the day they drink it.
IMO, corks are better, but from reading, there's more wine consumed than corks made.
furthermore, corks way of breathing is actually around the sides of the cork. with any temp changes, the cork will go in and out of the bottle some, imparting air. this is caused by pressure changes when temps fluctuate. the cork slides some to releave pressure. this is why you get legs of wine that come up the sides of older corks. from a documentary on them, a good cork will last between 15-18 yrs. they recommend for longer aging, uncorking, adding gas and then putting in a new cork.
a screw cap loses almost all the 02 in it in the first 3 months. meaning there is very little aging after that.
that said, we use some screw caps and mainly corks. i put them on by hand. i have a good grip and some have had trouble getting them off