Yet another vote for Yellowstone if you can only do one.
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2010 Outback 230RS Toybox, 5390# UVW, 6800# Loaded Not yet camped in Hawaii, 2 Canada Provinces, & 2 Territories I can't be lost because I don't care where this lovely road is going
There truly is so much more to do and see at Yellowstone for sure. We spent a week there and drove every road - so much diversity as it is so large. Unless you are really into hiking or photo work a couple of full days at the South Rim is sufficient to cover it one end to the other. (We do both)
God bless, be safe and enjoy!
DW of 50 great years, Barbara
2nd Best Friend-"Aussie" Terrier, Sadie
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Between the two, I would also choose Yellowstone because of the diversity (wildlife, geysers, mudpots, canyons, etc.). BUT, since you will be in southwestern CO, you could drive on up to ARCHES N.P. & CANYONLANDS N.P. near MOAB. Then head for Yellowstone N.P. That way, you could save Grand Canyon, Zion, Bryce Canyon National Parks and other great locations nearby for another great trip later. Have a great trip.