Which is better is a very tough question to answer, I'd really look at the
national parks website and explore the sites of both parks and see which looks more interesting to you.
If you are looking for more things to do beyond the one main stop, then I'd probably go with the Grand Canyon. I previously mentioned Zion, but there are lots of other possibilities. You've got Antelope Canyon, Walnut Canyon, the Sedona area, Petrified Forest/Painted Desert, or even Monument Valley (although that might be a bit far with 3 days). The Hoover Dam and Route 66 are also options on the way. Of course, you wouldn't have time for all of those things, but you'd have lots to pick from.
Death Valley is a bit more removed from other destinations - although it is a perfect location to include on a loop with San Francisco, Yosemite, and the Coast for a trip when you have more time. However, if DV is more appealing to you, another possibility would be to include Death Valley as part of a
Series of Daytrips from Vegas, returning to Vegas each night.
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