My breakdown opinion...
July
- More Crowded
- More locations likely available for fishing (great option if certain areas of the park are raining)
- Warmer weather
- Great Surface fly action with Terrestials
September
- Less Crowded
- Chance of Snow
- Cheaper Hotel rates
- Easier camping opportunities
- Chance to fish spawning brookies in Firehole and other high water areas.
- Chance to fish spawning Browns potentially.
- More potential interactions with wildlife on back country rivers. (especially grizzlies)