cost: We estimated the total costs before we started with our first child, and they would end up costing about the same as disposable diapers, however we have used them for 2 kids now :-)
potty training: Both our kids were potty trained at the age of 2, with little extraordinary effort, meaning the effort was constantly more since we have to clean the diapers. Indeed scraping poop from cloth is a good incentive for yourself to get them trained, so we combined the diapers with (but not very strict) the use "elimination communication" ("baby zindelijkheids communicatie" in Dutch). Simply put, we tried to keep diapers clean by holding the baby over the potty when we thought they were going to do nr 1 or nr 2.
My thoughts on why cloth diapers are better is this: the kids feel discomfort very clearly and immediately after the action, while the technologically superior disposable diapers are simply too comfortable to get this immediate action-response they need to learn by themselves, and indeed the first signs they were ready for full time potty training was that they started telling us the diaper was freshly made dirty.
environment: While honestly this is mostly based on my personal logic and not so much on science, but I cannot imagine cloth diapers being as bad for the environment as disposable diapers made from plastic and chemicals, event taking into consideration they need to be washed.