You should be able to use the track command like so:
track 1 interface Dialer0 ip routing
Then you have two default routes like so:
ip route 0.0.0.0 0.0.0.0 Dialer0 track 1
ip route 0.0.0.0 0.0.0.0 Dialer1 250
Alternatively you could use active tracking using IP sla to ping out of a certain interface and track the responses.
