The cleaning power is roughly proportional to the PSI rating multiplied by the GPM. While the higher pressure might not be necessary for the tasks you are considering, the added cleaning power means you'll get the job done faster with the more expensive unit. Keep in mind that you can always add distance or lower engine rotation to reduce pressure (back away from the surface), but you can't add pressure that the unit doesn't produce.
I have tried to use an old 1500 psi unit that I already had to clean a concrete driveway, but just purchased the 3000 psi machine from a home store, and the difference is amazing. My evaluation was that the extra $100 above the 2600 psi unit was well worth the hours saved. And the Honda engine gets very good reviews from most of what I've read.
I would suggest also getting the surface cleaning attachment, particularly for your wood deck. It's not really powerful enough for a really dirty concrete drive, but would surmise that it is sufficient for the wood (you don't want that high of pressure for the wood). For cleaning concrete surfaces, I highly recommend the Turbo nozzle; it takes the 0 deg pattern and spins it in a 15-25 deg (or so) pattern so that you get the dislodging pressure of the 0 degree, but in a pattern size that allows efficient cleaning.