Kobe won't be too different, you would only need a daytrip max. Compared to Osaka, Kobe's harbour is much cleaner and less industrialized. You could get to Kobe early in the day on the way to Osaka and hang out there for a while before heading to Osaka for the evening.
In the end don't try and pack your itinerary too full too make sure you can enjoy the whole trip fully, you may think you might get bored in the same city for a while. But you will be happily surprised at how much there is to do.