If you want any sort of peace of mind, "guarantee", or whatever about uptime. Go with a T1. Some providers will even give you a SLA (uunet) on it, which is nonexistant with DSL.
If you call the telco and say your T1 is down, they generally take it seriously. If you call and say your DSL is down they tend to laugh and you and tell you to join the crowd.
T1's are seen as business critical lines, as DSL is just joe-consumer stuff and no one cares if their down for 8 hours.
Do NOT go with DSL for a reliable means of transport. It WILL come back to haunt you, and you'll just be told when you bitch you should have went with a T1 and it's your own damn fault. (hell, that's what I'd say.)
-Phil