I took a plane ride oround Mt. MCinnly and I can tell you what we saw was spectacular but I will warn you the 90% of the time the mountain is surrounded by clouds and fog and getting close may be difficult. The mountain peaks leading up to Denali offer plenty of scenery though. As far as a helicopter or plane? You will have limited visibility in a plane depending on where you sit. This also holds true for a helicopter based on the copter's size and configuration (which you may not know what you're getting until they know how many people they're taking).
I would go on either as both are enjoyable. If you've never riden in a helicopter and can afford the extra cash then go that route as the pilot has more options when it comes to holding one position for site seeing.