None of them are great at stairs, it's hit or miss. In trying to decide for myself I extensively researched both these units. I spoke with all of people, including maytronics and techs who have worked at Leslie's pool supply.
From what I've been told, Polaris breaks down a lot, and it takes a while to get fixed sometimes. Also, it's not as good on walls because it has wheels instead of tracks. Also, the standard filter doesn't get sand or algae well.
i was going to get the s300i, but decided against it after talking to maytronics directly.
First, the only difference between the s300 and the s300i is that the 300i has more options on the Bluetooth. On the s300i you can set waterline only, wall only or deep clean where as you don't have those extra options on the 300. The only other difference is warranty. They both have 2 year warranties, but the s300i covers wearables in the second year. Those small differences weren't worth the $350 difference in price to me
Also, the 2017 Nautilis pro with powerstream is the same exact unit as the s300 but it doesn't come with caddy.
After speaking with maytronics for over 45 minutes and going over all the differences of the units I was considering, I went with the Dolphin Oasis z5. It has a 3 year warranty, includes caddy and remote. Best part was it was only $975 after rebate. I got it on epicpoolsupply and used a $25 coupon I found online (poolreview25) The warranty the biggest decision maker for me since these units can be expensive to repair. The rep also told me that any dealer can service it, but its almost always going to be faster to send any dolphin unit directly to them because the have the parts and its a 24-48 hour turnaround plus 2 days each way for shipping.
hope this helps
