I am running Method Double Standard wheels, 18x9 with -12 offset. I love them. The clearance around the caliper is about the same as the factory wheels It does look tight, but the factory put 18's on the truck as original equipment. Some advantages of 18 inch wheels are:
Pricing is more reasonable.
Offroad, your wheels are better protected and your tires better conform to obstacles such as rocks. They can also be aired down more safely than a tire with a shorter sidewall.
Weight - more often than not, the smaller diameter wheel/tire combo (with overall same total tire diameter) will be lighter so you have less rotating mass
More sidewall will generally give a bit softer ride (more flex in the sidewall)
Better fuel economy than the 20 inch equivalent
Less weight equates to less suspension wear
As for the bigger diameter rims, I would think there are also some advantages such as:
Better handling due to less sidewall flex
Ability to run larger aftermarket brakes
I would say if you are going for a vehicle that you primarily run on the street, then a 20 might give you better on road characteristics and better handling. However, that advantage is quickly reduced depending on what tire you run.
We often run tires that look really good, but really don't make a lot of sense for road driving. In the end, it is simply a matter of preference and how much you are willing to sacrifice for the look you want. I hate being practical...
