I'd say there are 3 main factors that make up what you are calling "bandwidth quality" - Settlement, Peering, and Network Capacity.
1 - Settlement - Unless you are buying transit from one of the big boys - IE MCI/UUNet, Sprint, Qwest, Level3, AT&T, Savvis, XO, or another true "tier-1" carrier, you aren't buying "settlement free" bandwidth. Whoever you are buying from has to hand it off to someone else in order to "settle" your data transmission. The more providers you settle to, generally the faster your traffic will reach it's destination, since providers with more settlement routes are able to send your traffic over one providers network more of the time, as long as they have their routing set up properly.
2 - Peering - Peering is similar to settlement, but for our example here we'll define peering as a direct connection to another provider for the purpose of exchanging data without using a 3rd party network. Good examples of this are ISP's that have links to large public peering points, such as PAIX, MAE-West, MAE-East, NYIIX, etc. Anyone connected to one of these public peering points has the ability to peer with any other provider connected to the same peering point. Many tier-2 providers settle a lot of their traffic this way to recuce costs - for example, Peer-1 and He.net. Almost any decent size city has some type of public peering location, and the more of them you are connected to, the more opportunities you have to peer with other providers. There is also private peering, where two providers would establish a private link with one another for the same purpose without ever going through a public exchange, since public peering points can get congested easily at times.
3 - Network Capacity - This is most commonly thought of as being the "pipe size" that any particular provider has going in to a specific area, although there is a bit more involved than that. I would define it to not only inlclude the "pipe size", but also to include quality of their network / switching gear, technology used to transmit data, as well as a few other things. This particular area is the most difficult to ascertain, since you won't always know what type of equipment a provider is using. Just be wary - even if a provider has an OC-48, that doesn't mean the rest of their network infrastructure is able to handle that kind of bandwidth.
This is a subject that would require much more space than we have here to adequately discuss. I'd welcome posts from any other members who have anything to add (or detract
) from what i've put out above.