joyrider
If your server is on, then you will get mail more quickly using smtp than pop.
If your server is off, then mail will be delivered when it comes back on, and there will/may be delays with pop or smtp in that case.
If you have frequent power failures then you should really be using a large enough UPS that will keep your server on for the duration. Mail will always get through then, and that's what your users want isn't it ?
You also don't risk damaging your hardware or file system with frequent uncontrolled power outages.