I've been to Death Valley several times camping. Some great hiking, but not if you want to do it as a day trip from LV. 4.5 hours of driving is a big push for 2-3 hours of being in Death Valley. But - if you go, Furnace Creek is where the visitors center is.. Scotty's castle is not too far north of there. Badwater is not too far south of there. We also enjoyed the Borax mining site.
If you had to choose between Death Valley and Red Rock - I'd do Red Rock Canyon - you need reservations to drive the loop with starting timed entry. But it's worth it. Lots of good short hikes off the loop, stunning scenery and one of my favorite visitors centers. (The view is framed perfectly from inside the visitors center.)
Earlier this year we camped in Red Rock Canyon and did a side trip to Valley of Fire. You *could* do both in one day. Again, if you had to choose, Red Rock would be my choice.
Red Rock is literally right outside Vegas. We went into town to casinos for dinner bargains on 2 of the nights.
When would you be doing this - it's still pretty hot in Death Valley. But by mid-October it's better.