Ron,
I know what you're saying, that some chems are more suitable for xjetting....but I feel that's too much of a shotgun approach on a deck. I like the more precise control I get with the 12v pump sprayer. Plus, it's a whole lot easier to meter your chems just right. I have several helpers, and it's easier to teach them to mix the chems to the proper strength right in the bucket. All my deck chems are mixed at 1 plastic measuring cup per gallon of water, so when they make different sized batches they don't have to stop and try to recalculate different mixtures. It's a lot different when you have hired help who may be more prone to mistakes. That's just my personal feelings.
Now for house washing, I wouldn't be without my xjet.
I also xjet chems on the trucks I wash. I only do five cube vans, and probably won't be doing more any time soon, so the speed factor doesn't bother me. I don't two step, I only use a product called Atlas. Cube vans are super easy to wash, and easy to brush.
In any case, I think everyone out here washing should have one in their arsenal.