Having one provider leaves with you a possible outage and 100% of your customers to be offline. Having multiple providers enables you to use the GREAT thing we call BGP (Border Gateway Protocol) to in lamest terms LINK the lines. If line (1) where to die or become overloaded with latency issues line (2) would take further traffic like load balancing.
Also line (1) and line (2) will each have different peering points with other isps this enables you as the client for a "fast route" to your customers.
Lets say line (1) is MCI and line (2) is Level3 If a majority of your customers are MCI and these lines are BGP'd and the network engineer setup the routers correctly it would take the MCI path so your customers end point to his colocated server has less latency and less network hops thus faster connection into the network and out of the network. Also MCI has peering with other providers so if MCI is peered with GLOBIX or SAVVIS in which they are and your customers are using a DSL or line from SAVVIS they will take the MCI path in.
Also just for reference MCI and Level3 are peered together but each offer unique routing policys.
As stated before its best to choose a provider with multiple providers to the internet so all your customers will find a faster route in and limit the amount of downtime on the network side while the provider will be using BGP (Multi-Homed Network).
This is proboly confusing and I'm proboly lost myself but I just woke up! hope this helps a little.
location is almost as important as the quality of the provider you buy your bandwidth from. If you have a lot of customers surfing from asia, you want to have globalcrossing or at&t bandwidth instead of say cogent or level3. If you have a lot of road runner cable modem home users, you may want to directly peer with road runner.
No one size fits all.
And quality is mainly a factor of the provider's national (or international) backbone, which really can only be guaged through asking around and looking at test results on your own. Generally speaking, COGENT is one of those that gets less than stellar feed back though they seem to always be improving and for their price (usualy half anyone else) its usualy good enough for most except quality hosting (preimum) and voip/gaming type applications.
Oh and quality is mainly defined as how long your customer has to sit and wait to do something. That could be measured in 'latency', total throughput, or even 'how often its down' depending on the carrier. File transfers like ftp dont really care about latency, and things like VOIP/gaming care very much about latency. Premium/quality is where the latency usualy gets to be the best.
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