Why purchase a T1 / T3 when SDSL is cheaper? - T1 circuits are transported over a more robust channel.  DSL is designed to be a lower cost alternative to the more traditional data communications product, such as Frame Relay and T1's.  One of the ways this lower cost is achieved is by having DSL circuits share more of the data transport infrastructure.  With 1.5 Mbps SDSL, your data traffic shares the connection with other DSL circuits running at slower speeds. On the other hand, a T1 line is dedicated to transporting only T1 traffic.

A T1 transfers data between two points at 1.544 Mbps symmetrically, and is ideal for customers who need a high-speed connectivity to the Internet or as one link in their virtual or real private network. A T1 connection provides roughly 60 times more data than a normal residential modem. It is also extremely reliable, much more reliable than an analog modem. Depending on what they are doing, a T1 line can generally handle quite a few people. For general browsing, hundreds of users are easily able to share a T1 line comfortably. Most small Internet providers have a T1 (or a fractional T1) line as their connection to the Internet. In addition, a T1 connection is a dedicated link and unlike ISDN and analog modem connections, the T1 connection is up 24 hours a day.