What type of main drain do you have?
In my pool, I have two separated circular main drains with covers that stick up about 3/8" above the plaster surface. It's enough that if my S300i hits the main drain dead center, it will get stuck on the drain for about 30-60secs before executing a power thrust turn. In the M series, you get the extra 3rd brush and the ability to do split tread turning (true 90 deg turns). This makes the M series much less susceptible to hangups. People with channel drains tend to have much harder time with hang ups.
I'm always more inclined to shop locally for something as complicated as a bot. As I said in a previous post, I found an excellent shop with a knowledgeable repair staff so I feel it was well worth the small extra cost. When you purchase "online" it's not truly an "online" purchase, you'll be calling the shop you are buying from, giving them a credit card number and having them ship the bot to you. I live out in a more rural part of town and UPS/FedEx does an awesome job of ensuring that all of my orders are drop-kicked for the last mile or so! I once ordered a large (65") UHD TV from Costco and had to refuse the shipment drop off because the box had obvious forklift damage (even though the box clearly indicates to NOT use a forklift or pallet jack)....I made the driver open the box in the truck BEFORE he pulled it out and the front screen of the TV was smashed! I simply can't get good delivery service where I am so i would never trust a $1200+ robot to be shipped without damage...you might have better shipping where you live.
Anyway, I'm all for local brick & mortar if you can find one. Also, if the purchase does not include the caddy, then I feel that's a no-go. I love my caddy. Make sure when you're ordering "online" that you are getting exactly the same unit you would get by walking into a shop and buying it direct. Sometimes these online deals are a $100 cheaper because they are not actually selling you everything...they take out the caddy of the S300i and then sell you the robot for $100 less and you think you're getting a deal...