Which end? in Montana

I've been lucky enough to spend a fairly large amount of time in Idaho and
Montana, over the years, in the times between June1st and Sept 1st.

I've heard, but never experienced, good things about the fishing in April
and Sept.

This season I hope to include one of the above months in my plans, but I'm
having trouble deciding which end of the season to try. If you have
experience fishing Montana ( I'm thinking mainly of the Dillon area for
April and Big Horn -East Yellowstone for Sept ) at these times and places,
I'd love for you to share your experiences, to help me decide.

Notes:
Good, bad, or indifferent, I equate fly fishing with hatches and rising
fish. I just this minute returned from a 5 mile drive to 'local water"
where I went so far as to don waders before I admitted to myself that
sloshing around looking a a piece of yarn wasn't interesting enough to
justify the rest of rigging up, even though I would nearly certainly have
caught some fish.

And my interest in fishing isn't a passion like I used to have for duck
hunting, interest deep enough to justify suffering :-) ... if it's too damn
cold, I don't care if he's rising steady and 24" ... I just can't get into
being REAL cold ( or hot, or otherwise uncomfortable ) to catch a fish. I
have fished and enjoyed it, on snowy days but iced guides and such are for
guys more dedicated than myself ... that's duck weather G

So experiences on the likelyhood of good insect emergences is primary in
what I hope to obtain via this inquiry. Add weather reports and I might be
well enough armed to make up my mind

Thanks, in advance