CFL Foiler said..
Greg's wind range charts are accurate. Make your decision based on the chart for sure. If you think your typical wind range is going to be 15 with gusts to 20 I would vote for the 4.2. The 5.3 can be ridden in those conditions, its actually just sheet and go in those conditions, but you will be very lit and probably not ideal for learning. The 4.2 is much easier to manage size wise and they all can be pumped well which is similar to a wing.
I love my 5.3 though. Its awesome for 10-15 conditions. Its a rocket ship in 15-20, still fun but probably a bit much for learning. I am so happy with the wind range that I actually skipped a size and went with the 3.2 for my next size down.
You skip all the way to the 3.2 Kanaha?
I have the 3.4 S3 and am considering skipping the 4.2 and getting the 5.3 for 10-17 mph. How does the 5.3 handle the lulls in gutless, humid, warm wind? And how does it wad / pack? I am sure it will feel massive, but compared to an aluula inflatible 5m wing that I would otherwise use in these conditions, the S3 would be way lighter and half the price.