IMHO, the gas powered models are far superior. Remember to look at both PSI and GPM in comparing models. Some cheaper gas models have as high a PSI as a more expensive model, but a lower GPM, which makes any task take longer. But if budget is more important than time, they will clean just as well. Also, I suggest looking for a unit with removable spray tips set at different angles, as opposed to the single adjustable tip. I'm personally a big fan of Honda's small engines.
I've used pressure washers for a long time. My summer job growing up was washing boat hulls (among other things) at the local marina. I've never seen a water filter used on a pressure washer. I'm sure there is some value in it, but I've never had any problems with the units I've used, and they had very heavy use. Of course, they were more heavy duty commercial models.
Also, a common mistake when using a pressure washer is leaving the engine running too long with no water flow. People like to spray for a while, and then put down the wand and start scrubbing a particular area, spraying chemical, or get a sandwich, all with the engine and pump still running. Its fine to leave it running with no flow for a little while, but if its going to be minutes you need to shut it down. Otherwise you can tear up the pump. On a related note, never let your idiot brother in law borrow your pressure washer.
Good luck finding what you need.
