Adaptive or offset is definitely better for longevity and better overall temps since you're going to be in idle states a lot; it also helps prevent electromigration. If this is a system you're planning on keeping for over 2 years, I would recommend adaptive. Idling at maximum voltage is a bit silly because the temps and power usage will be higher than is necessary, and you're also causing unnecessary electromigration by having manual voltage - I've come to have a disdain for manual voltage because the temps creep up over time. There's no reason to idle at 1.28V or what have you, when IVB and Haswell can idle near 1V (IVB) or sub 1V (Haswell) and lower power consumption/temps.
Anyway, i'd say adaptive is the way to go. Consult your manual, but you normally have to put your boost vcore target in your settings, and Haswell's iVR will take care of the rest during idle states. You can also try offset voltage, but it is a LOT trickier. Generally speaking, adaptive voltage with asus motherboards is pretty straightforward and easy, hopefully that will be the case for you as well. If you try offset, plan on spending a couple of hours or more tinkering with it. Offset is a pain in the neck to dial in.
The only word of caution is that in many cases, offset or adaptive have the potential to have the proper boost vcore, but then will idle at a vcore which is too low. If that happens, you will BSOD. I've found this problem myself using offset, so be aware that you may have to play with it a bit - adaptive should be fairly easy to set up, though.