Hi. Sorry I'm just now getting back to you. Have been out of town.
The answer to your question really depends on what you want to do and what you enjoy. Denali is an awesome sight and you are likely to see quite a bit of wildlife if you take the time to go into the park. If this is likely to be your only trip to Alaska, I would say don't miss Denali. However, it is a National Park and it will be busy and commercialized as such.
Kodiak doesn't have as much commercial packaged stuff to do, although there is plenty to keep you occupied. We are an ocean based community, so think kayaking, saltwater and freshwater fishing, sealions, puffins, whales, etc. There's also lots of hiking and beachcombing. You don't say what month you are traveling, but Kodiak didn't get it's nickname of the Emerald Isle by being arid. If you come here plan to encounter rain and make up your mind it's not going to stop you from getting out.
If you have any other questions, please let me know. I should be in town for awhile now.