The gel pad are just what it says...a pad. It will go from too-cold to warm in an hour or more.

The evap coolers are meant to last overnight. You have some control over the temp on an evaporative cooler and can (through trial and error) find a setting that you like and hold it at that temperature for the duration of the night. Loose control to your comfort zone. It works. I have one, it can get quite cool in dry conditions because there is a lot of evaporation. It would be less cool in a humid environment because you would get less evaporation. Like I said, by trial and error you can find a reasonably comfortable cool setting that will last overnight.