Basically with robots, you have to figure which features you absolutely need and then find out what the suppliers in your area are stocking. Maytronics pretty much supplies the entire robot industry with various models and platforms creating their own product line as well as manufacturing the brand name labels for major pool equipment suppliers (i.e., Pentair's robot is actually made by Maytronics). So there's often not much difference in the name...
Also, I would suggest you buy local from a brick & mortar shop so that you have a local repair option. Many people will sometimes try to save a hundred bucks or so buying online and then have to ship their robots out when trouble strikes. I recently went with an S300i for the options and because the shop I bought it from is an authorized Dolphin repair shop and is 3 miles from my home.
This would be my first pool cleaner and due to the complexity of the structure of my pool, I want to make sure the robot can clean most of the complex surface area. There is not much flat area in the deep end, so I need something that can understand lots of inclines.
Does the S100 understand the layout of your pool or does it just run a pre-programmed pattern?
Note regarding the above model listing, there are some inaccuracies in it. Dolphin Nautilus/Nautilus Plus 2017 models are not twins to the S100 or S200. Dolphin Cayman = S100= Dolphin E10= Doheny Saturn. S200=Active 20, Doheny Discovery=Dolphin Triton Plus. Among others.
S100 has one brush and one year warranty, S200 has two brushes and two year warranty. Not much else is different.
Have you called around to local stores and asked about a demo model? At least two of the shops I talked with local to me were willing to let me demo the unit I was interested in for a week to see if I liked it at no cost. It might help you decide.