I sailed on the Mercury in Sept 99 on the last Alaska trip that year. I thought the ship was great, a good choice for Alaska. She has lots of glass and spots to watch the scenery go by if the weather keeps you inside. This time of year the weather can be real changeable, just be ready for anything. Day can be cool and nights cold, but crowds should be smaller everywhere.
You will find bargains ashore and aboard ship as everything is closing down for the season. If the price is right I'd go for it, Alaska weather is a guessing game any month.