Both are great. If you go to Valley of Fire don't miss the Petroglyphs and Mouses Tank. Great stories about both if your interested in history.
If Las Vegas is your home base for the trip consider travel time. Death Valley is a full day trip. Getting there seeing it and getting back is 8-10 hours so that is your whole day and early evening. You could do Valley of Fire in 4-6 hours round trip so you would have a full evening after getting back.
Good luck and have a great time either way! almost 9 years ago
Valley of fire was awesome, going back again this year. Here are some photos from last year, http://www.flickr.com/photos/cazillo/sets/72157624651464705/ almost 9 years ago