Makes no difference which you use, the larger sizes (512K and 1024K) are just the 256K bios repeated to 'fill the gap' if your chip size supports it.
You might as well flash the 512K on one of the banks of your chip, then you have a spare 512K bank to try a different type if you don't like the features of the evox bios.
Edit: Just check which version you have as well. I would recommend
www.xbox-scene.com as an excellent resource. I also noticed this thread, which suggests you may need to build your own bios if you have a 1.6 version Xbox.
http://team-xecuter.com/forums/showthread.php?t=4894