Both of those write a single ISO to your USB stick, while Ventoy allows you to store numerous ISOs in a folder on the stick and choose which to use at runtime. Also, you can store other files like normal with the remaining space on your stick.
Rufus and BalenaEtcher are both programs for flashing an image to a disk. Ventoy is flashed onto the disk itself (into a small EFI partition), then the rest of the disk is just a regular file system, where you drag and drop a group of ISOs, then pick between them on boot.
Those let you write one image to a USB stick. With Ventoy you write the bootable part once, and plop as many ISOs on there as you want. You get one bootable device where you can select from a list of ISOs.