Depending on the router, you can look at metrics on the T1 connection to determine if it truely is saturated or not.
One thing that may assist you is to do ping tests to determine latency and/or packet loss. If your router isn't showing errors and its not pushing your T1s capacity and you're getting packet loss and/or high latency from your T1 carrier... you may want to confront them with the data.
It may be that your DSL provider is more *reliable* than your T1 provider... not necessarily more bandwidth.
Mick.