just to be clear these two choices are presented by the bios in the boot menu to ask which device to boot from, they arnt really unibeast options, unibeast is the App that creates the USB installer.
So When you say you only 1 choice, what is that choice exactly ? screenshot might be helpful
If you don't get the 2 choices at boot time, there can be only two possibilities. 1. The bios itself is different, have you upgraded to the latest bios, and checked all the bios settings ? 2. The USB drive has not been prepared incorrectly or is defective, I would suggest trying a different USB drive, erasing and recreating it as per the guide.
My general advice would be to take a step back, and start fresh, go step by step through the guide. Assume nothing, if something is unclear in the guide ask for clarification. Also can you Write down exactly what you do at each step, this will be very important to help work out where you went wrong (if it still doest not work)
Kiwi