Depends on what you plan to do with your Ex.
If you plan to off road, 18's are likely your better bet.
* More tire = less chance of bending the rim
* 18" Tires cheaper to replace ($199ea vs. $306ea for Michelin Latitude Tour from Tirerack.com, for apples to apples comparison)
* 18" Rims are cheaper to replace and easier to find. ($200ea vs $350-painted 20's to$400 polished 20's, source: Originalwheels.com)
Get 20's if you want:
* The looks of a bigger wheel/less tire
* Slightly better handling (but worse ride)
* 7% drop in Torque (
http://www.4x4abc.com/4WD101/math_wheels.html), decrease stopping force (see previous link for more info) and small penalty in MPG (Heavier wheel and tire combo)
Also, my experience with polished rims is that they do not stay pretty for long. After while, the coating wears off and it looks ugly. Painted wheels will likely look better longer..