NPE-400/512M can take full BGP tables, if you need that. I haven't worked on a SMS-1800 in years; SMS was an DSL/ATM aggregation chassis, if that's what you need. 7206 is more multi-function, used as a large edge box or core for small/medium-sized enterprises, small edge for ISPs, etc.
SMS has moved from a DSL/ATM aggregation chassis When the company announced that its latest upgrade, the XFE Forwarding Engine Module, would provide native Gigabit Ethernet support. Redback chose to go around the backplane and deliver Gigabit Ethernet packets directly from the module to the Internet. I can say at this point, that SMS can out-perform 7206 both as a multifunction router and at the same time, provide the native DSL/ATM aggregation.