New (RBK853) vs. old (RBR50) - Experiences?

I only have 1 AX supporting device and don't use it much. Mostly for testing.

I have about 30 devices on my network, wired and wireless. 

Both work pretty well for my needs and size of home. 

Orbi AC has matured over the years and seems fairly stable now days, sans some minor device naming issues. AX is still maturing and NG is working on that. There is a problem with NEST controllers on Orbi AX. NG is aware and is working on it. There are some router web page features that are seen on Orbi AC that are not seen on the Orbi AX. Items like disable SSID broadcast, something we have been asking for. Since this would help get some IoT devices connected better. Daisy chain, which seems to be automatic now on Orbi AX so may not need a user button for this. Same with Fast Roaming and Beamforming. These seem to be running on a as needed or automatic basis. 

Also NG and Orbi seem to be working with a different CTS value from what I'm told. 64 is the new default which in some cases seems to cause some problems for users. Me included initiially. Historically CTS has been 2347 for a long while. I guess with Orbi AX, NG seems to find that 64 is the new default. I have been using this and side the NEST problem and my mac book disconnecting in one spot in my home when I have both RBS online, NG seems to want to push the 64 value for CTS. Users can of course change this as they feel they need to. I feel there is some tuning still needed here by NG. 

Orbi AC RBS are not wirelessly compatible with Orbi AX. I've seen some information that points to NG not going to change this. They will work if they are ethernet connected to the Orbi AX system however. The RBSY outdoor can connect however you have to install the extender mode FW on to the RBSY to make that happen. NGs EX7500 and EX7700s work with Orbi AX as well so if someone needs additional wifi somewhere and can't find a RBS to buy, the EX will work in a pinch. 

Orbi AX seems to have great speeds and same coverage as Orbi AC if placed well throughtout the home and conditions are good. 

I like that the Orbi AX has the ability to connect to LAG supporting modems for highspeed internet sharing and speeds, especially on 1Gb ISP services. 

IF you don't have any AX supporting devices and your Orbi AC is working for you, you might consider holding off for now. Unless you find some stellar deal out there on a AX  system, might give one a try and see for your self. 

My two cents.