Regulated
I have both and my mech sits in my case most of the time, but its great backup.
I have to say regulated because you get the ease of having longer battery life, more capabilities for tanks, RDA, RTA, or carto. You can see your battery life, ohms, voltage, and wattage. You can give it 5 clicks and throw it in your bag/car/pocket and not really have to worry about it.
I can fully verify I have put my IPV3 through some gruesome snow conditions up in the mountains boarding and it has never failed to fire once. That being said I will always have my Mech because its the most reliable backup.