Some of this may be driven by the installer and what they had available or were encouraged to push by the manufacture(s) they work with. In addition, there are many (installers) who still get caught-up on "pool turnover" and think they need to push X-amount of water daily to keep it clean which we know isn't the case. I agree that being a single speed might be the biggest concern. At least with a 2-speed you can save energy costs and keep the flow much slower until you really want it pushed harder. That's what I do. My pump is on low 99.9% of the time. So when the time comes to replace the motor, I would definitely consider a 2 speed. If you expand your system later or have any additional plumbing needs (waterfall, fountains, etc) the 2 HP might still be valuable. If not, you could drop your HP some, but still look for a 2 speed in the long-run.