The 2020 models run WebOS5 instead of 4.5 but its not clear of the differences, the younger one will receive updates later than the older though. The E9 also supports DTS audio, although with a Sonos bar that isn't really useful.
If you plan on pairing the TV with its own sound bar then much of what you are getting with the E9 is wasted. The design of the TV is more aesthetically pleasing than the CX but you will probably just be better off getting the more modern BX or CX. Just beware that because of the time of year right now you'll be overpaying for the BX and CX compared to the E9, since those are close to the start of the pricing cycles and are currently priced high, whilst the E9 is over a year old now and cheaper.
BX vs CX is just case a few small differences moistly to picture processing. You can compare the difference at rtings.com