I have a similarly sized office, and I addressed the limitations of our 1.5Mbps DSL by bringing in a business class cable modem (16down, 2up). It's hooked to a dual WAN router, where I customized the iptables so that the highest traffic protocols (http/https, RDC, FTP) go out the Cable, all other traffic hits the default route of the DSL.
As to whether or not the T1 speed is > ADSL, we don't know how fast your ADSL is. As long as they both perform properly, 1.5Mbps is the same over either connection. Biggest difference is that a T1 is usually provided with a higher availability SLA.
BTW,when you're looking at IP traffic, the # of T1 channels isn't part of the consideration. The channels are about voice calls only.
