Would you recommend root canal or an extraction on an infected molar k9 tooth that has coronal leakage and soft dentin. Thanks.?

Depends. Whether it is a molar or a canine tooth either of these procedures can be done depending on the restorative prognosis of the tooth. Coronal leakage and soft dentin can be fixed by placing a new restoration. Root canal may be needed if the decay extends into/near the pulp or if their is evidence of infection clinically or on an x-ray. If the tooth is not fixable then an implant may be an option.
No. Canine teeth are the longest rooted teeth in your mouth and are usually the last to fail. Sounds like by coronal leakage, you mean that there is an ill fitting crown that may be decaying thus softening the dentin. If the tooth is restorable, it is an important and funtional tooth that should be restored if possible. Another good option is to extract the tooth and place an implant. Consult dds.