DSL Modem verses Router

A side from firewalls which is better a DSL Modem or a DSL Router?
Reason being, we switched to DSL from ISDN for cost savings. Now we see performance go down considerably. We are running Low Speed DSL because high is not available yet. We are supose to have 128k Up and 384k Down.
The only difference is that we went from a ISDN router to a DSL modem.
The signal from our ISP shows good connection both ways.
Does anyone have any ideas?
Thanks
Bobb@billygoat.com