Yellowstone National Park is much larger in area. It is more than 8 times larger than Rocky Mountain National Park.
Yellowstone National Park is inscribed in the UNESCO World Heritage list. Rocky Mountain National Park is not a UNESCO World Heritage site.
Yellowstone is famous for its Grizzly bears. There are no grizzly bears in Rocky Mountain National Park.
Both National Parks have American Black bears.
There are American Bisons in Yellowstone National Park. Rocky Mountain National Park has no population of bison.
Yellowstone National Park is famous for its thermal features and wildlife. Rocky Mountain National Park is famous for its incredible mountain views, alpine meadows and lakes.
Yellowstone is older than Rocky Mountain National Park. Yellowstone - 1 March 1872 Rocky Mountain NP - 26 January 1915
Yellowstone National Park - Eagle Peak (11,372 ft/3,466 m) Rocky Mountain National Park - Longs Peak (14,259 ft/4,346 m)
Yellowstone NP North Rim trail, Mt Washburn hike, Avalanche Peak trail, etc. Rocky Mountain NP Bear Lake Hike, Alberta Falls Trail, Alpine Ridge Trail, Emerald Lake Trail, etc.
Yellowstone National Park - Wyoming, Montana, and Idaho Rocky Mountain National Park - Colorado