The biggest difference between different models of window washers is the squeegee. A soft squeegee reduces the risk of lines left on the glass, and you do not have to press as hard when you want to reach the bottom of a window. It's a good idea to make sure the squeegee is made of a material more similar to windscreen wipers than thick rubber.
Read about the battery life of the model. If you have a lot of windows and want to be able to clean them all at once, you should make sure that the window cleaner you choose is able to do the job on one charge. Some models will tell you how much battery you have left. This is not really necessary, but it helps you assess whether you will have time to do another window and avoid standing up on a ladder with a suddenly discharged battery.
Some brands have their own washing fluid, but you can also make your own. We've found a great DIY-recipe if you want to give it a go!