I just spent 8 days in Yellowstone mid-July and spent 8 in Glacier last summer. If it’s possible to just choose one I would suggest that, wayyyyy too much park to only spend a few days there. After 8 days in Yellowstone I felt like we hadn’t even scratched the surface even with the northeast section being closed. The crowds weren’t nearly as bad as expected, I’m assuming a lot of people cancelled trips after the flood expecting the park to be completely closed.
Glacier was a madhouse when we went last year, we were lucky to have a going to the sun road pass and lucked into a first-come-first-serve spot in a campground in the park. It was worth the effort to get the camp site, had to be there at 5:30 am to get in line, the ranger then comes out and tells you how many spots will be opening, they then free everyone to run through the campground to find sites without a ticket to claim, absolute shit show.
Avalanche lake is a crowded trail but was awesome.
Logan pass is a cool stop, go super early if you want to try to park.
Many Glaciers is an amazing area, saw a grizzly over there. (the park ranger who was maybe 120lbs soaking wet, literally chased it off with a hand full of rocks and his bear spray to end the traffic jam
)
Polebridge was cool, stop for the huckleberry bear claw. A lot more solitude up past that area and a ton of big whitetail bucks.
Yellowstone - if you’re fishing the park can’t be beat. The only places we really saw people were in Haden Valley drooling over bison, and at fairy falls. Outside of that on hikes we didn’t see many people at all which was nice. Saw probably 8 bears, a grizzly with cubs (saw her 3 times), countless elk, bighorn sheep, bison, coyotes, eagles, no wolves though.
Cascade Lake - Nice hike, great lake to fish.
Fire hole river - incredible fishing
Galiten River/ Bighorn pass - Black bear central
Grebe Lake - awesome views, decent fishing
Washburn mountain - bighorns everywhere, beautiful 5g tower at the top.