I need to get my 2400 sq.ft., 2-story house pressure washed, and had a few services visit the house to provide quotes. The quotes are widely different so I need some advice to decide. Service A said he will use a 3000psi jet, put up a tall scaffold to stand on while washing, and charge $600 total. Service B said he will do a soft wash using a low-pressure jet (< 200psi pretty much same as the house water supply pressure), use a pole to reach the second story, and charge $300 total. Is Service A's method overkill and/or potentially damaging to the house? Does Service B's method sound sufficient given the amount of mold/mildew on the house is moderate? Both services use the same chemical (bleach).
Ideally you would use the lowest pressure that will be effective, to minimize the chance of damage. A soft wash generally focuses on letting the detergent dwell and do the work, while a harder wash can make do with fewer (or no) chemicals. Some jobs clearly require one or the other, but siding is somewhere in the middle.
Assuming that both operators are skilled the high pressure guy has the potential advantage of using fewer chemicals, which is nice for plants and animals. The greater height from the scaffolding should also make it easier to aim the spray down, which tends to work better with siding than up. On the other hand, the high pressure stream is more likely to force water into places you'd rather not have it. Got any poorly flashed windows?
The low pressure guy is potentially less dangerous to your house, potentially more so to your garden if he's loading up on the chemicals. Using a pole puts him further from the work. That being said, you really care how the house looks from the ground. It doesn't sound like there are any features that couldn't be cleaned with the pole. Are there gargoyles or corbels that need the close up touch?
In the end you'll have to decide who is more trustworthy, whether that's by asking questions or seeing their previous work. Neither sounds too outlandish, though neither sounds extremely compelling. Two hundred psi sounds very low to remove mildew that's grown into the siding while three thousand sounds fairly high for a house.