Glacier NP has beautiful vistas but Yellowstone NP has much more unusual geologic features. Mammoth Hot Springs looks like giant stone layers of a white wedding cake. The Yellowstone Canyon and the painted rock colors are out of this world! Old Faithful and other geysers pop up periodically out of the ground.
I didn't dare take even little hikes at Glacier since I was afraid of grizzlies appearing out of nowhere. I wouldn't trust bear spray since you never know in a panic, where the wind is coming from. The red jammer bus tours would have been nice, though I didn't take one. At Yellowstone, I was always near my vehicle or a building.
If you do go to Glacier NP, consider taking the Amtrak *Empire Builder* train from Seattle, Chicago or points in between. It stops right at East Glacier Lodge. For a few sections, such as Seattle to Havre, Montana, there are NPS guides who narrate the scenery in the observation car from May to mid September. I know, because I have been one of those guides!