Well, BellSouth specifically disclaims any minimum level of service on their webpage, but that's the real difference between the two tiers - with T1, you get guarantees, which is why it costs a whole lot more than DSL or cable. E.g., Covad's standard T1 SLA guarantees 99.99% monthly uptime, which translates to less than five minutes of down time per month, and they guarantee packet-level performance in terms of maximum transit time. And good luck getting that from DSL or cable - you may have uptime of 100% at a blazingly fast rate so far, but BellSouth will be the first to tell you they don't owe you a thing if you have a week-long period where your connection is slightly faster than a 14.4K modem, which they would if you had a real dedicated connection - you pay more because if your service falls below that minimum level, you have a contract that spells out exactly what your remedies are, so there's no argument about that sort of thing when there is an interruption, no trying to sweet-talk them into giving you some sort of a credit. And hopefully someone in your previous office was making reference to that SLA with the problems you mention you were having before.
This is not to say that there's no place for business-class DSL or cable in the world - I'm sure there are lots of people it's perfectly suited for - but there are trade-offs that come with that low price. If you just need it for interoffice email and internet access for your users, then it's probably just fine. If you're running your own e-commerce business through your own dedicated servers, such that every second of downtime costs you money for lost sales, or something like that...you may want to skip the cable connection ;)