WEll, most will say that the high pressure wash is the way to go. But I have heard and seen lots to the contrary. It really depends on the condition of your siding. If you are just changing the color and the siding is just faded and dirty the pressure wash won't hurt. However, if you are repainting because it is needed to reprotect the house you should really inspect the siding closely. Dry rot will get knock out of place and leave a great deal of priming to do. If there are lots of cracks the pressure will drive water under the siding and after you paint the water will want to come out...through the paint. Be careful and if you are not sure check with a REALLY good (probably really expensive) pro. If you are going to spend the money spend some time and do it correctly once. It should last 10 years.