I'm a little drunk, but I think I can help you out.
USUALLY, not always and I'm not looking at the site you're on, but usually when it says 14 -> 18mm joint, it usually means is comes with a joint that fits into an 14mm stem or slide on your bong, and the other side, the female part, where the nail is 18mm to be able to fit 18mm attachments. When you have more attachments, you usually want to add an attachment with more "chug" and this requires more air flow usually more than a 14mm to be able to clear it with ease. Again it depends what size of piece you're using.
As far as 19mm, I have no idea. Pieces in the United States usually come in 14.4, and 18.8... perhaps they rounded up? I'd assume a 18mm nail would fit a 19mm dome, but not a 14mm nail. I could be wrong though.
As far as quality, definitely get a TI nail over a quartz or glass nail. They retain heat much better, and you don't have to worry about them breaking. If you'd like, I can PM you a site that has many nails and domes to offer. They're located out of California, and they have some of the best glass / accessories around, they even have a whole section of their site dedicated to concentrate smoking. Bad thing is, they can be quite pricey but I'll tell you it's worth the extra buck to get a quality rig than a shitty one.