[villagetinker]

I have done both. I have power washed the driveway, lanai and birdcage several time myself, and continue to these myself. I use a Ryobi electric power washer which works very well. I have done the house twice, then used a contractor. I was disappointed when I ended up with very noticeable stains after the contractor finished. After I called him back, and he came out to look, it was determined that the staining was due to irrigation water. He recommended SNO WHITE, which I purchased at ACE Hardware. I applied this myself, and just hosed off, and all id good.
My point is this, if you do the job yourself, you can see what is happening, and make adjustments. If you hire a contractor you may end up doing some work yourself anyway.
Now everyone is going to ask who did I use, this was Enger's, and I was very pleased with the job in general, but what happened was the stains that were barely visible prior to cleaned became an eyesore after. I was expecting this to be made correct, but it was explained that this happens some time. While I would tend to use Enger's again in the future, I would have a detailed discussion on my expectations of the final result. For the time being, I will probably do my own, when the water restrictions allow......