Should I buy an RBK853 or RBK843S

Cannot speak to effectiveness of the S version.  I have the rbk853.   I bought it about a month ago.  

old setup was ATT.  Eero pro mesh.   House is about 4500 sq ft brick house.  I had drops regularly at end of house and no support to garage or yard.   When it did connect it was SLOW, maybe 5mbs.  The other challenge is my three TVs could not get enough throughput to run concurrently.

today I have xfinity, I  own my 3.1 modem.   I connected the Orbi 853.   I have strong signal everywhere inside AND outside the house.  One network name (ssid) services my 50+ iot, television, Sonos, fire tv et.al.  Everything has been operating without a lag and working outside in the yard I do not note a drop off in speed.  I am sure it degregates, but on my computer, phone or iPad it does effect my work.

after the system is on for 48 hours it stabilizes and delivers as promises.   The 853 is overkill I believe,  could have gotten good coverage from 1 satellite.  But nothing exeeds like having more than you need.

I will caution you that if you have to replace the modem after initial setup, you will have to do a reset to Orbi.  That means setting up the system again.   Tried to reinitializethe system for a few hours before I reset to factory.   Much quicker and less frustrating.

Good luck.  The system has massive throughput with the 1.2g coming in.   I see 800 to 925 coming across my house and no buffering to devices.