I can't tell if you're agreeing or disagreeing with me. Your tone sounds to me like you're disagreeing, but you're then proceeding to restate what you quote me as saying.
He's disagreeing with you about the reliability of mirrored devices.
Mirrored boot devices is a joke and is something of a sales tactic to me. I didn't want to comment on it but after Cyberjock jumped in, well...
Honestly, not all motherboards will properly support this so called automatic failover. Typically when you setup your MB you tell it which device is the boot device which means you likely have to pick one of the USB devices, or SATA drives and place things in a boot order sequence. So lets just say that works fine, now you have a primary boot device and at some point in time it fails. The question is, do you really think it will fail and not lock up your machine? Seriously, only proper intelligent hardware can pull off something like that. Here is a perfect example... Your normal boot flash starts the bootup process and then crashes about half way into it with some fatal trap. How in the world will the mirrored device magically fix this? I'm all ears for a realistic answer that is completely free of operator interaction.
Seriously, operation interaction is needed and once you have figured out the failed flash drive, you remove it and possibly reconfigure the machine BIOS to get things running again. To test the failover you just cannot just pull one of the flash drives out, you need to have a no shit failure. This is why I don't like mirrored boot devices, it's false hope in my mind. Backup the configuration file and be ready to rebuild the boot device if needed. When it comes to a SSD, odds are in favor you will not have a failed boot device due to a hardware failure of the SSD. You can bet that you will have a USB flash drive failure eventually, they have a finite life span.
So we are back to a single boot device, SSD is my preference and I'll get there after FreeNAS 10 hits the streets because there is no reason to upset my system while it's working fine. Also I'll have to blow the dust bunnies out when that time comes so I'll have to open the case anyway.
At the OP, I hope you post what you end up doing.