EX6120 in Bridge Mode – Should Backhaul Be Connected to Both Bands or Just One?

First off. Disabling the "radios" doesn't. If they were disabled, it couldn't connect back to the router. You just disabled the ssid broadcast. It doesn't impact performance or stop the actual wifi from broadcasting. Not much use to do other than looks nicer from a standard scanner. 

Next. Even though the 2.4ghz is showing a better signal it doesn't mean its faster. 2.4ghz is a slow network and sensitive to interference. Its really slow. Test the speed on the hardwired device with each band to see what one actually provides the higher throughput. 

IF it was me, I'd use the 5ghz. Much less interference. Tends to be much faster unless its at the very edge of its range. So a little testing is in your best bet. 

I'd also probably leave the ssid's broadcasting. Disabling them doesn't change anything other than your thoughts on it so you might as well have them broadcasting in case you want to put a few devices (like IoT devices) on the extender.