FortiAP advice

They both have 3 radios (2.4Ghz, 5Ghz, scanning). Major difference is that 831F supports 8x8 MU-MIMO and is therefore equipped with 5Gb Ethernet port.

If you think you the theoretical 2.5Gbps per AP is not sufficient for your use case, then 831-F is the only option. Just ensure your switches are quipped with 5GE ports.

You might also consider 431G if you plan on using WiFi 6e.

All FortiAP specs can be compared at https://www.fortinet.com/content/dam/fortinet/assets/data-sheets/fortiap-series.pdf

The question this poses.
What do you want to be able to cover.
- Voice
- Simple surfing
- CAD
and so on.

What could also be interssant how long the AP should be operated.

As bpozdena_FTNT already mentioned are the 431G/433G an alternative and ask Wifi 6E

High density wi-fi is served best by many small cells with narrow bands. This avoids co-channel interference and over-crowding on the APs.

I would not try and solve a high-density requirement by using a few 8x8 APs here and there. You would be best suited just deploying many more 4x4 APs and reducing their transmit power and using 20Mhz channels to reduce overlap.

You would really do best to consult with a wi-fi certified Fortinet partner to do a proper site survey and predictive layout for your office. There are so many variables when it comes to wi-fi you really can't get all of your answers on a forum like this.

I did the mistake of purchasing 60 - U431-F and have had a horrible experience compared to the 431-F and other models.  Firmware is extremely picky and can cause the U431-F to constantly reboot.  When I upgraded the FortiGate to 7.0.9 and U431-F to 7.0.0, which is currently the only release in 7.0 for U431-F, the AP would not authenticate Radius clients.  I had to downgrade to 6.2.5.  I am using 431-F in high density areas and it works great when the power settings are turned down low allowing devices to roam between APs.  I am currently running v 7.0.5 on 431-F without any reported issues.  I will not purchase U431-F anymore.