ADSL is asymmetrical -- the downstream speed is usually much faster than the upstream. And adsl is usually cheap (often under $50 per month) 1.5mbps downstream and 128kbps upstream is a pretty standard speed.
SDSL is symetrical -- the upstream and downstream speeds are the same. That is what we have here - 2 sdsl lines, both 1.5mbps upstream and downstream. You pay a premium for that fast upstream speed, tho. It's about $400 a month per line for us.
Reliability is pretty much the same between the two. It's the upstream speed that is the big difference. And the support -sdsl is generally aimed at businesses, so the support people for sdsl providers are more than just script readers.
SDSL is symmetrical -- upload