Power usage should vary insignificantly from one mobo to another, especially considering the impact of other components, like the hard drive(s) and/or SSD used.
As to boot time, isn't UEFI supposed to be faster? I'm not sure, as I haven't used one yet.
it is not only motherboard which will be reponsible for fast booting
if you are using a z68 + sandy bridge system.
then if you use SSD hard disk and install your OS on SSD harddisk and use normal sata harddisk as normal backup of your files only then this configuration will be best for you.
but if you are using normal sata for your OS and backup both. then you will not see any major difference.