It is very hot in the Mojave in Summer. The Preserve is huge, and what do you want to do there? I live in California, and I have visited most of the Preserve. It is stark severe desert with few improvements. Notable features are Mitchell Caverns, Kelso Dunes, various abandoned mines, Cima Dome, some volcanic features, and rugged mountains and wide basins.
Joshua Tree and Death Valley are adjacent, and more interesting destinations. Joshua Tree is at a higher altitude and slightly less hot.